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<h1>Node.js v16.16.0 documentation</h1>
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<li><a href="#modules-ecmascript-modules">Modules: ECMAScript modules</a>
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<li><a href="#introduction">Introduction</a></li>
<li><a href="#enabling">Enabling</a></li>
<li><a href="#packages">Packages</a></li>
<li><a href="#import-specifiers"><code>import</code> Specifiers</a>
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<li><a href="#terminology">Terminology</a></li>
<li><a href="#mandatory-file-extensions">Mandatory file extensions</a></li>
<li><a href="#urls">URLs</a>
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<li><a href="#file-urls"><code>file:</code> URLs</a></li>
<li><a href="#data-imports"><code>data:</code> Imports</a></li>
<li><a href="#node-imports"><code>node:</code> Imports</a></li>
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<li><span class="stability_1"><a href="#import-assertions">Import assertions</a></span></li>
<li><a href="#builtin-modules">Builtin modules</a></li>
<li><a href="#import-expressions"><code>import()</code> expressions</a></li>
<li><a href="#importmeta"><code>import.meta</code></a>
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<li><a href="#importmetaurl"><code>import.meta.url</code></a></li>
<li><span class="stability_1"><a href="#importmetaresolvespecifier-parent"><code>import.meta.resolve(specifier[, parent])</code></a></span></li>
</ul>
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<li><a href="#interoperability-with-commonjs">Interoperability with CommonJS</a>
<ul>
<li><a href="#import-statements"><code>import</code> statements</a></li>
<li><a href="#require"><code>require</code></a></li>
<li><a href="#commonjs-namespaces">CommonJS Namespaces</a></li>
<li><a href="#differences-between-es-modules-and-commonjs">Differences between ES modules and CommonJS</a>
<ul>
<li><a href="#no-require-exports-or-moduleexports">No <code>require</code>, <code>exports</code> or <code>module.exports</code></a></li>
<li><a href="#no-__filename-or-__dirname">No <code>__filename</code> or <code>__dirname</code></a></li>
<li><a href="#no-native-module-loading">No Native Module Loading</a></li>
<li><a href="#no-requireresolve">No <code>require.resolve</code></a></li>
<li><a href="#no-node_path">No <code>NODE_PATH</code></a></li>
<li><a href="#no-requireextensions">No <code>require.extensions</code></a></li>
<li><a href="#no-requirecache">No <code>require.cache</code></a></li>
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<li><span class="stability_1"><a href="#json-modules">JSON modules</a></span></li>
<li><span class="stability_1"><a href="#wasm-modules">Wasm modules</a></span></li>
<li><span class="stability_1"><a href="#top-level-await">Top-level <code>await</code></a></span></li>
<li><span class="stability_1"><a href="#https-and-http-imports">HTTPS and HTTP imports</a></span>
<ul>
<li><a href="#imports-are-limited-to-http1">Imports are limited to HTTP/1</a></li>
<li><a href="#http-is-limited-to-loopback-addresses">HTTP is limited to loopback addresses</a></li>
<li><a href="#authentication-is-never-sent-to-the-destination-server">Authentication is never sent to the destination server.</a></li>
<li><a href="#cors-is-never-checked-on-the-destination-server">CORS is never checked on the destination server</a></li>
<li><a href="#cannot-load-non-network-dependencies">Cannot load non-network dependencies</a></li>
<li><a href="#network-based-loading-is-not-enabled-by-default">Network-based loading is not enabled by default</a></li>
</ul>
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<li><span class="stability_1"><a href="#loaders">Loaders</a></span>
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<li><a href="#hooks">Hooks</a>
<ul>
<li><a href="#resolvespecifier-context-defaultresolve"><code>resolve(specifier, context, defaultResolve)</code></a></li>
<li><a href="#loadurl-context-defaultload"><code>load(url, context, defaultLoad)</code></a></li>
<li><a href="#globalpreload"><code>globalPreload()</code></a></li>
</ul>
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<li><a href="#examples">Examples</a>
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<li><a href="#https-loader">HTTPS loader</a></li>
<li><a href="#transpiler-loader">Transpiler loader</a></li>
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<li><a href="#resolution-algorithm">Resolution algorithm</a>
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<li><a href="#features">Features</a></li>
<li><a href="#resolver-algorithm">Resolver algorithm</a></li>
<li><a href="#resolver-algorithm-specification">Resolver Algorithm Specification</a></li>
<li><span class="stability_1"><a href="#customizing-esm-specifier-resolution-algorithm">Customizing ESM specifier resolution algorithm</a></span></li>
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<li><a href="https://nodejs.org/docs/latest-v14.x/api/esm.html">14.x <b>LTS</b></a></li>
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<li><a href="https://nodejs.org/docs/latest-v12.x/api/esm.html">12.x <b>LTS</b></a></li>
<li><a href="https://nodejs.org/docs/latest-v11.x/api/esm.html">11.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v10.x/api/esm.html">10.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v9.x/api/esm.html">9.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v8.x/api/esm.html">8.x</a></li></ol></div>
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</header>
<details id="toc" open><summary>Table of contents</summary><ul>
<li><a href="#modules-ecmascript-modules">Modules: ECMAScript modules</a>
<ul>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#enabling">Enabling</a></li>
<li><a href="#packages">Packages</a></li>
<li><a href="#import-specifiers"><code>import</code> Specifiers</a>
<ul>
<li><a href="#terminology">Terminology</a></li>
<li><a href="#mandatory-file-extensions">Mandatory file extensions</a></li>
<li><a href="#urls">URLs</a>
<ul>
<li><a href="#file-urls"><code>file:</code> URLs</a></li>
<li><a href="#data-imports"><code>data:</code> Imports</a></li>
<li><a href="#node-imports"><code>node:</code> Imports</a></li>
</ul>
</li>
</ul>
</li>
<li><span class="stability_1"><a href="#import-assertions">Import assertions</a></span></li>
<li><a href="#builtin-modules">Builtin modules</a></li>
<li><a href="#import-expressions"><code>import()</code> expressions</a></li>
<li><a href="#importmeta"><code>import.meta</code></a>
<ul>
<li><a href="#importmetaurl"><code>import.meta.url</code></a></li>
<li><span class="stability_1"><a href="#importmetaresolvespecifier-parent"><code>import.meta.resolve(specifier[, parent])</code></a></span></li>
</ul>
</li>
<li><a href="#interoperability-with-commonjs">Interoperability with CommonJS</a>
<ul>
<li><a href="#import-statements"><code>import</code> statements</a></li>
<li><a href="#require"><code>require</code></a></li>
<li><a href="#commonjs-namespaces">CommonJS Namespaces</a></li>
<li><a href="#differences-between-es-modules-and-commonjs">Differences between ES modules and CommonJS</a>
<ul>
<li><a href="#no-require-exports-or-moduleexports">No <code>require</code>, <code>exports</code> or <code>module.exports</code></a></li>
<li><a href="#no-__filename-or-__dirname">No <code>__filename</code> or <code>__dirname</code></a></li>
<li><a href="#no-native-module-loading">No Native Module Loading</a></li>
<li><a href="#no-requireresolve">No <code>require.resolve</code></a></li>
<li><a href="#no-node_path">No <code>NODE_PATH</code></a></li>
<li><a href="#no-requireextensions">No <code>require.extensions</code></a></li>
<li><a href="#no-requirecache">No <code>require.cache</code></a></li>
</ul>
</li>
</ul>
</li>
<li><span class="stability_1"><a href="#json-modules">JSON modules</a></span></li>
<li><span class="stability_1"><a href="#wasm-modules">Wasm modules</a></span></li>
<li><span class="stability_1"><a href="#top-level-await">Top-level <code>await</code></a></span></li>
<li><span class="stability_1"><a href="#https-and-http-imports">HTTPS and HTTP imports</a></span>
<ul>
<li><a href="#imports-are-limited-to-http1">Imports are limited to HTTP/1</a></li>
<li><a href="#http-is-limited-to-loopback-addresses">HTTP is limited to loopback addresses</a></li>
<li><a href="#authentication-is-never-sent-to-the-destination-server">Authentication is never sent to the destination server.</a></li>
<li><a href="#cors-is-never-checked-on-the-destination-server">CORS is never checked on the destination server</a></li>
<li><a href="#cannot-load-non-network-dependencies">Cannot load non-network dependencies</a></li>
<li><a href="#network-based-loading-is-not-enabled-by-default">Network-based loading is not enabled by default</a></li>
</ul>
</li>
<li><span class="stability_1"><a href="#loaders">Loaders</a></span>
<ul>
<li><a href="#hooks">Hooks</a>
<ul>
<li><a href="#resolvespecifier-context-defaultresolve"><code>resolve(specifier, context, defaultResolve)</code></a></li>
<li><a href="#loadurl-context-defaultload"><code>load(url, context, defaultLoad)</code></a></li>
<li><a href="#globalpreload"><code>globalPreload()</code></a></li>
</ul>
</li>
<li><a href="#examples">Examples</a>
<ul>
<li><a href="#https-loader">HTTPS loader</a></li>
<li><a href="#transpiler-loader">Transpiler loader</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#resolution-algorithm">Resolution algorithm</a>
<ul>
<li><a href="#features">Features</a></li>
<li><a href="#resolver-algorithm">Resolver algorithm</a></li>
<li><a href="#resolver-algorithm-specification">Resolver Algorithm Specification</a></li>
<li><span class="stability_1"><a href="#customizing-esm-specifier-resolution-algorithm">Customizing ESM specifier resolution algorithm</a></span></li>
</ul>
</li>
</ul>
</li>
</ul></details>
<div id="apicontent">
<h2>Modules: ECMAScript modules<span><a class="mark" href="#modules-ecmascript-modules" id="modules-ecmascript-modules">#</a></span><a aria-hidden="true" class="legacy" id="esm_modules_ecmascript_modules"></a></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v16.14.0</td>
<td><p>Add support for import assertions.</p></td></tr>
<tr><td>v16.12.0</td>
<td><p>Consolidate loader hooks, removed <code>getFormat</code>, <code>getSource</code>, <code>transformSource</code>, and <code>getGlobalPreloadCode</code> hooks added <code>load</code> and <code>globalPreload</code> hooks allowed returning <code>format</code> from either <code>resolve</code> or <code>load</code> hooks.</p></td></tr>
<tr><td>v14.8.0</td>
<td><p>Unflag Top-Level Await.</p></td></tr>
<tr><td>v15.3.0, v12.22.0</td>
<td><p>Stabilize modules implementation.</p></td></tr>
<tr><td>v14.13.0, v12.20.0</td>
<td><p>Support for detection of CommonJS named exports.</p></td></tr>
<tr><td>v14.0.0, v13.14.0, v12.20.0</td>
<td><p>Remove experimental modules warning.</p></td></tr>
<tr><td>v13.2.0, v12.17.0</td>
<td><p>Loading ECMAScript modules no longer requires a command-line flag.</p></td></tr>
<tr><td>v12.0.0</td>
<td><p>Add support for ES modules using <code>.js</code> file extension via <code>package.json</code> <code>"type"</code> field.</p></td></tr>
<tr><td>v8.5.0</td>
<td><p><span>Added in: v8.5.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<p></p><div class="api_stability api_stability_2"><a href="documentation.html#stability-index">Stability: 2</a> - Stable</div><p></p>
<section><h3>Introduction<span><a class="mark" href="#introduction" id="introduction">#</a></span><a aria-hidden="true" class="legacy" id="esm_introduction"></a></h3>
<p>ECMAScript modules are <a href="https://tc39.github.io/ecma262/#sec-modules">the official standard format</a> to package JavaScript
code for reuse. Modules are defined using a variety of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import"><code>import</code></a> and
<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export"><code>export</code></a> statements.</p>
<p>The following example of an ES module exports a function:</p>
<pre><code class="language-js"><span class="hljs-comment">// addTwo.mjs</span>
<span class="hljs-keyword">function</span> <span class="hljs-title function_">addTwo</span>(<span class="hljs-params">num</span>) {
<span class="hljs-keyword">return</span> num + <span class="hljs-number">2</span>;
}
<span class="hljs-keyword">export</span> { addTwo };</code></pre>
<p>The following example of an ES module imports the function from <code>addTwo.mjs</code>:</p>
<pre><code class="language-js"><span class="hljs-comment">// app.mjs</span>
<span class="hljs-keyword">import</span> { addTwo } <span class="hljs-keyword">from</span> <span class="hljs-string">'./addTwo.mjs'</span>;
<span class="hljs-comment">// Prints: 6</span>
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">addTwo</span>(<span class="hljs-number">4</span>));</code></pre>
<p>Node.js fully supports ECMAScript modules as they are currently specified and
provides interoperability between them and its original module format,
<a href="modules.html">CommonJS</a>.</p>
<!-- Anchors to make sure old links find a target -->
<p><i id="esm_package_json_type_field"></i><i id="esm_package_scope_and_file_extensions"></i><i id="esm_input_type_flag"></i></p>
</section><section><h3>Enabling<span><a class="mark" href="#enabling" id="enabling">#</a></span><a aria-hidden="true" class="legacy" id="esm_enabling"></a></h3>
<p>Node.js has two module systems: <a href="modules.html">CommonJS</a> modules and ECMAScript modules.</p>
<p>Authors can tell Node.js to use the ECMAScript modules loader
via the <code>.mjs</code> file extension, the <code>package.json</code> <a href="packages.html#type"><code>"type"</code></a> field, or the
<a href="cli.html#--input-typetype"><code>--input-type</code></a> flag. Outside of those cases, Node.js will use the CommonJS
module loader. See <a href="packages.html#determining-module-system">Determining module system</a> for more details.</p>
<!-- Anchors to make sure old links find a target -->
<p><i id="esm_package_entry_points"></i><i id="esm_main_entry_point_export"></i><i id="esm_subpath_exports"></i><i id="esm_package_exports_fallbacks"></i><i id="esm_exports_sugar"></i><i id="esm_conditional_exports"></i><i id="esm_nested_conditions"></i><i id="esm_self_referencing_a_package_using_its_name"></i><i id="esm_internal_package_imports"></i><i id="esm_dual_commonjs_es_module_packages"></i><i id="esm_dual_package_hazard"></i><i id="esm_writing_dual_packages_while_avoiding_or_minimizing_hazards"></i><i id="esm_approach_1_use_an_es_module_wrapper"></i><i id="esm_approach_2_isolate_state"></i></p>
</section><section><h3>Packages<span><a class="mark" href="#packages" id="packages">#</a></span><a aria-hidden="true" class="legacy" id="esm_packages"></a></h3>
<p>This section was moved to <a href="packages.html">Modules: Packages</a>.</p>
</section><section><h3><code>import</code> Specifiers<span><a class="mark" href="#import-specifiers" id="import-specifiers">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_specifiers"></a></h3>
<h4>Terminology<span><a class="mark" href="#terminology" id="terminology">#</a></span><a aria-hidden="true" class="legacy" id="esm_terminology"></a></h4>
<p>The <em>specifier</em> of an <code>import</code> statement is the string after the <code>from</code> keyword,
e.g. <code>'path'</code> in <code>import { sep } from 'path'</code>. Specifiers are also used in
<code>export from</code> statements, and as the argument to an <code>import()</code> expression.</p>
<p>There are three types of specifiers:</p>
<ul>
<li>
<p><em>Relative specifiers</em> like <code>'./startup.js'</code> or <code>'../config.mjs'</code>. They refer
to a path relative to the location of the importing file. <em>The file extension
is always necessary for these.</em></p>
</li>
<li>
<p><em>Bare specifiers</em> like <code>'some-package'</code> or <code>'some-package/shuffle'</code>. They can
refer to the main entry point of a package by the package name, or a
specific feature module within a package prefixed by the package name as per
the examples respectively. <em>Including the file extension is only necessary
for packages without an <a href="packages.html#exports"><code>"exports"</code></a> field.</em></p>
</li>
<li>
<p><em>Absolute specifiers</em> like <code>'file:///opt/nodejs/config.js'</code>. They refer
directly and explicitly to a full path.</p>
</li>
</ul>
<p>Bare specifier resolutions are handled by the <a href="#resolver-algorithm-specification">Node.js module resolution
algorithm</a>. All other specifier resolutions are always only resolved with
the standard relative <a href="https://url.spec.whatwg.org/">URL</a> resolution semantics.</p>
<p>Like in CommonJS, module files within packages can be accessed by appending a
path to the package name unless the package’s <a href="packages.html#nodejs-packagejson-field-definitions"><code>package.json</code></a> contains an
<a href="packages.html#exports"><code>"exports"</code></a> field, in which case files within packages can only be accessed
via the paths defined in <a href="packages.html#exports"><code>"exports"</code></a>.</p>
<p>For details on these package resolution rules that apply to bare specifiers in
the Node.js module resolution, see the <a href="packages.html">packages documentation</a>.</p>
<h4>Mandatory file extensions<span><a class="mark" href="#mandatory-file-extensions" id="mandatory-file-extensions">#</a></span><a aria-hidden="true" class="legacy" id="esm_mandatory_file_extensions"></a></h4>
<p>A file extension must be provided when using the <code>import</code> keyword to resolve
relative or absolute specifiers. Directory indexes (e.g. <code>'./startup/index.js'</code>)
must also be fully specified.</p>
<p>This behavior matches how <code>import</code> behaves in browser environments, assuming a
typically configured server.</p>
<h4>URLs<span><a class="mark" href="#urls" id="urls">#</a></span><a aria-hidden="true" class="legacy" id="esm_urls"></a></h4>
<p>ES modules are resolved and cached as URLs. This means that special characters
must be <a href="url.html#percent-encoding-in-urls">percent-encoded</a>, such as <code>#</code> with <code>%23</code> and <code>?</code> with <code>%3F</code>.</p>
<p><code>file:</code>, <code>node:</code>, and <code>data:</code> URL schemes are supported. A specifier like
<code>'https://example.com/app.js'</code> is not supported natively in Node.js unless using
a <a href="#https-loader">custom HTTPS loader</a>.</p>
<h5><code>file:</code> URLs<span><a class="mark" href="#file-urls" id="file-urls">#</a></span><a aria-hidden="true" class="legacy" id="esm_file_urls"></a></h5>
<p>Modules are loaded multiple times if the <code>import</code> specifier used to resolve
them has a different query or fragment.</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> <span class="hljs-string">'./foo.mjs?query=1'</span>; <span class="hljs-comment">// loads ./foo.mjs with query of "?query=1"</span>
<span class="hljs-keyword">import</span> <span class="hljs-string">'./foo.mjs?query=2'</span>; <span class="hljs-comment">// loads ./foo.mjs with query of "?query=2"</span></code></pre>
<p>The volume root may be referenced via <code>/</code>, <code>//</code> or <code>file:///</code>. Given the
differences between <a href="https://url.spec.whatwg.org/">URL</a> and path resolution (such as percent encoding
details), it is recommended to use <a href="url.html#urlpathtofileurlpath">url.pathToFileURL</a> when importing a path.</p>
<h5><code>data:</code> Imports<span><a class="mark" href="#data-imports" id="data-imports">#</a></span><a aria-hidden="true" class="legacy" id="esm_data_imports"></a></h5>
<div class="api_metadata">
<span>Added in: v12.10.0</span>
</div>
<p><a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs"><code>data:</code> URLs</a> are supported for importing with the following MIME types:</p>
<ul>
<li><code>text/javascript</code> for ES Modules</li>
<li><code>application/json</code> for JSON</li>
<li><code>application/wasm</code> for Wasm</li>
</ul>
<p><code>data:</code> URLs only resolve <a href="#terminology"><em>Bare specifiers</em></a> for builtin modules
and <a href="#terminology"><em>Absolute specifiers</em></a>. Resolving
<a href="#terminology"><em>Relative specifiers</em></a> does not work because <code>data:</code> is not a
<a href="https://url.spec.whatwg.org/#special-scheme">special scheme</a>. For example, attempting to load <code>./foo</code>
from <code>data:text/javascript,import "./foo";</code> fails to resolve because there
is no concept of relative resolution for <code>data:</code> URLs. An example of a <code>data:</code>
URLs being used is:</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> <span class="hljs-string">'data:text/javascript,console.log("hello!");'</span>;
<span class="hljs-keyword">import</span> _ <span class="hljs-keyword">from</span> <span class="hljs-string">'data:application/json,"world!"'</span>;</code></pre>
<h5><code>node:</code> Imports<span><a class="mark" href="#node-imports" id="node-imports">#</a></span><a aria-hidden="true" class="legacy" id="esm_node_imports"></a></h5>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v16.0.0</td>
<td><p>Added <code>node:</code> import support to <code>require(...)</code>.</p></td></tr>
<tr><td>v14.13.1, v12.20.0</td>
<td><p><span>Added in: v14.13.1, v12.20.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<p><code>node:</code> URLs are supported as an alternative means to load Node.js builtin
modules. This URL scheme allows for builtin modules to be referenced by valid
absolute URL strings.</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs/promises'</span>;</code></pre>
</section><section><h3>Import assertions<span><a class="mark" href="#import-assertions" id="import-assertions">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_assertions"></a></h3>
<div class="api_metadata">
<span>Added in: v16.14.0</span>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<p>The <a href="https://github.com/tc39/proposal-import-assertions">Import Assertions proposal</a> adds an inline syntax for module import
statements to pass on more information alongside the module specifier.</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> fooData <span class="hljs-keyword">from</span> <span class="hljs-string">'./foo.json'</span> assert { <span class="hljs-attr">type</span>: <span class="hljs-string">'json'</span> };
<span class="hljs-keyword">const</span> { <span class="hljs-attr">default</span>: barData } =
<span class="hljs-keyword">await</span> <span class="hljs-title function_">import</span>(<span class="hljs-string">'./bar.json'</span>, { <span class="hljs-attr">assert</span>: { <span class="hljs-attr">type</span>: <span class="hljs-string">'json'</span> } });</code></pre>
<p>Node.js supports the following <code>type</code> values, for which the assertion is
mandatory:</p>
<table><thead><tr><th>Assertion <code>type</code></th><th>Needed for</th></tr></thead><tbody><tr><td><code>'json'</code></td><td><a href="#json-modules">JSON modules</a></td></tr></tbody></table>
</section><section><h3>Builtin modules<span><a class="mark" href="#builtin-modules" id="builtin-modules">#</a></span><a aria-hidden="true" class="legacy" id="esm_builtin_modules"></a></h3>
<p><a href="modules.html#core-modules">Core modules</a> provide named exports of their public API. A
default export is also provided which is the value of the CommonJS exports.
The default export can be used for, among other things, modifying the named
exports. Named exports of builtin modules are updated only by calling
<a href="module.html#modulesyncbuiltinesmexports"><code>module.syncBuiltinESMExports()</code></a>.</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> <span class="hljs-title class_">EventEmitter</span> <span class="hljs-keyword">from</span> <span class="hljs-string">'events'</span>;
<span class="hljs-keyword">const</span> e = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>();</code></pre>
<pre><code class="language-js"><span class="hljs-keyword">import</span> { readFile } <span class="hljs-keyword">from</span> <span class="hljs-string">'fs'</span>;
<span class="hljs-title function_">readFile</span>(<span class="hljs-string">'./foo.txt'</span>, <span class="hljs-function">(<span class="hljs-params">err, source</span>) =></span> {
<span class="hljs-keyword">if</span> (err) {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(err);
} <span class="hljs-keyword">else</span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(source);
}
});</code></pre>
<pre><code class="language-js"><span class="hljs-keyword">import</span> fs, { readFileSync } <span class="hljs-keyword">from</span> <span class="hljs-string">'fs'</span>;
<span class="hljs-keyword">import</span> { syncBuiltinESMExports } <span class="hljs-keyword">from</span> <span class="hljs-string">'module'</span>;
<span class="hljs-keyword">import</span> { <span class="hljs-title class_">Buffer</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'buffer'</span>;
fs.<span class="hljs-property">readFileSync</span> = <span class="hljs-function">() =></span> <span class="hljs-title class_">Buffer</span>.<span class="hljs-title function_">from</span>(<span class="hljs-string">'Hello, ESM'</span>);
<span class="hljs-title function_">syncBuiltinESMExports</span>();
fs.<span class="hljs-property">readFileSync</span> === readFileSync;</code></pre>
</section><section><h3><code>import()</code> expressions<span><a class="mark" href="#import-expressions" id="import-expressions">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_expressions"></a></h3>
<p><a href="https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#Dynamic_Imports">Dynamic <code>import()</code></a> is supported in both CommonJS and ES modules. In CommonJS
modules it can be used to load ES modules.</p>
</section><section><h3><code>import.meta</code><span><a class="mark" href="#importmeta" id="importmeta">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_meta"></a></h3>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li>
</ul>
<p>The <code>import.meta</code> meta property is an <code>Object</code> that contains the following
properties.</p>
<h4><code>import.meta.url</code><span><a class="mark" href="#importmetaurl" id="importmetaurl">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_meta_url"></a></h4>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The absolute <code>file:</code> URL of the module.</li>
</ul>
<p>This is defined exactly the same as it is in browsers providing the URL of the
current module file.</p>
<p>This enables useful patterns such as relative file loading:</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> { readFileSync } <span class="hljs-keyword">from</span> <span class="hljs-string">'fs'</span>;
<span class="hljs-keyword">const</span> buffer = <span class="hljs-title function_">readFileSync</span>(<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(<span class="hljs-string">'./data.proto'</span>, <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>));</code></pre>
<h4><code>import.meta.resolve(specifier[, parent])</code><span><a class="mark" href="#importmetaresolvespecifier-parent" id="importmetaresolvespecifier-parent">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_meta_resolve_specifier_parent"></a></h4>
<!--
added:
- v13.9.0
- v12.16.2
changes:
- version: v16.2.0
pr-url: https://github.com/nodejs/node/pull/38587
description: Add support for WHATWG `URL` object to `parentURL` parameter.
-->
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<p>This feature is only available with the <code>--experimental-import-meta-resolve</code>
command flag enabled.</p>
<ul>
<li><code>specifier</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The module specifier to resolve relative to <code>parent</code>.</li>
<li><code>parent</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="url.html#the-whatwg-url-api" class="type"><URL></a> The absolute parent module URL to resolve from. If none
is specified, the value of <code>import.meta.url</code> is used as the default.</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise" class="type"><Promise></a></li>
</ul>
<p>Provides a module-relative resolution function scoped to each module, returning
the URL string.</p>
<!-- eslint-skip -->
<pre><code class="language-js"><span class="hljs-keyword">const</span> dependencyAsset = <span class="hljs-keyword">await</span> <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-string">'component-lib/asset.css'</span>);</code></pre>
<p><code>import.meta.resolve</code> also accepts a second argument which is the parent module
from which to resolve from:</p>
<!-- eslint-skip -->
<pre><code class="language-js"><span class="hljs-keyword">await</span> <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-string">'./dep'</span>, <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>);</code></pre>
<p>This function is asynchronous because the ES module resolver in Node.js is
allowed to be asynchronous.</p>
</section><section><h3>Interoperability with CommonJS<span><a class="mark" href="#interoperability-with-commonjs" id="interoperability-with-commonjs">#</a></span><a aria-hidden="true" class="legacy" id="esm_interoperability_with_commonjs"></a></h3>
<h4><code>import</code> statements<span><a class="mark" href="#import-statements" id="import-statements">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_statements"></a></h4>
<p>An <code>import</code> statement can reference an ES module or a CommonJS module.
<code>import</code> statements are permitted only in ES modules, but dynamic <a href="#import-expressions"><code>import()</code></a>
expressions are supported in CommonJS for loading ES modules.</p>
<p>When importing <a href="#commonjs-namespaces">CommonJS modules</a>, the
<code>module.exports</code> object is provided as the default export. Named exports may be
available, provided by static analysis as a convenience for better ecosystem
compatibility.</p>
<h4><code>require</code><span><a class="mark" href="#require" id="require">#</a></span><a aria-hidden="true" class="legacy" id="esm_require"></a></h4>
<p>The CommonJS module <code>require</code> always treats the files it references as CommonJS.</p>
<p>Using <code>require</code> to load an ES module is not supported because ES modules have
asynchronous execution. Instead, use <a href="#import-expressions"><code>import()</code></a> to load an ES module
from a CommonJS module.</p>
<h4>CommonJS Namespaces<span><a class="mark" href="#commonjs-namespaces" id="commonjs-namespaces">#</a></span><a aria-hidden="true" class="legacy" id="esm_commonjs_namespaces"></a></h4>
<p>CommonJS modules consist of a <code>module.exports</code> object which can be of any type.</p>
<p>When importing a CommonJS module, it can be reliably imported using the ES
module default import or its corresponding sugar syntax:</p>
<!-- eslint-disable no-duplicate-imports -->
<pre><code class="language-js"><span class="hljs-keyword">import</span> { <span class="hljs-keyword">default</span> <span class="hljs-keyword">as</span> cjs } <span class="hljs-keyword">from</span> <span class="hljs-string">'cjs'</span>;
<span class="hljs-comment">// The following import statement is "syntax sugar" (equivalent but sweeter)</span>
<span class="hljs-comment">// for `{ default as cjsSugar }` in the above import statement:</span>
<span class="hljs-keyword">import</span> cjsSugar <span class="hljs-keyword">from</span> <span class="hljs-string">'cjs'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(cjs);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(cjs === cjsSugar);
<span class="hljs-comment">// Prints:</span>
<span class="hljs-comment">// <module.exports></span>
<span class="hljs-comment">// true</span></code></pre>
<p>The ECMAScript Module Namespace representation of a CommonJS module is always
a namespace with a <code>default</code> export key pointing to the CommonJS
<code>module.exports</code> value.</p>
<p>This Module Namespace Exotic Object can be directly observed either when using
<code>import * as m from 'cjs'</code> or a dynamic import:</p>
<!-- eslint-skip -->
<pre><code class="language-js"><span class="hljs-keyword">import</span> * <span class="hljs-keyword">as</span> m <span class="hljs-keyword">from</span> <span class="hljs-string">'cjs'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(m);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(m === <span class="hljs-keyword">await</span> <span class="hljs-title function_">import</span>(<span class="hljs-string">'cjs'</span>));
<span class="hljs-comment">// Prints:</span>
<span class="hljs-comment">// [Module] { default: <module.exports> }</span>
<span class="hljs-comment">// true</span></code></pre>
<p>For better compatibility with existing usage in the JS ecosystem, Node.js
in addition attempts to determine the CommonJS named exports of every imported
CommonJS module to provide them as separate ES module exports using a static
analysis process.</p>
<p>For example, consider a CommonJS module written:</p>
<pre><code class="language-js cjs"><span class="hljs-comment">// cjs.cjs</span>
<span class="hljs-built_in">exports</span>.<span class="hljs-property">name</span> = <span class="hljs-string">'exported'</span>;</code></pre>
<p>The preceding module supports named imports in ES modules:</p>
<!-- eslint-disable no-duplicate-imports -->
<pre><code class="language-js"><span class="hljs-keyword">import</span> { name } <span class="hljs-keyword">from</span> <span class="hljs-string">'./cjs.cjs'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(name);
<span class="hljs-comment">// Prints: 'exported'</span>
<span class="hljs-keyword">import</span> cjs <span class="hljs-keyword">from</span> <span class="hljs-string">'./cjs.cjs'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(cjs);
<span class="hljs-comment">// Prints: { name: 'exported' }</span>
<span class="hljs-keyword">import</span> * <span class="hljs-keyword">as</span> m <span class="hljs-keyword">from</span> <span class="hljs-string">'./cjs.cjs'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(m);
<span class="hljs-comment">// Prints: [Module] { default: { name: 'exported' }, name: 'exported' }</span></code></pre>
<p>As can be seen from the last example of the Module Namespace Exotic Object being
logged, the <code>name</code> export is copied off of the <code>module.exports</code> object and set
directly on the ES module namespace when the module is imported.</p>
<p>Live binding updates or new exports added to <code>module.exports</code> are not detected
for these named exports.</p>
<p>The detection of named exports is based on common syntax patterns but does not
always correctly detect named exports. In these cases, using the default
import form described above can be a better option.</p>
<p>Named exports detection covers many common export patterns, reexport patterns
and build tool and transpiler outputs. See <a href="https://github.com/nodejs/cjs-module-lexer/tree/1.2.2">cjs-module-lexer</a> for the exact
semantics implemented.</p>
<h4>Differences between ES modules and CommonJS<span><a class="mark" href="#differences-between-es-modules-and-commonjs" id="differences-between-es-modules-and-commonjs">#</a></span><a aria-hidden="true" class="legacy" id="esm_differences_between_es_modules_and_commonjs"></a></h4>
<h5>No <code>require</code>, <code>exports</code> or <code>module.exports</code><span><a class="mark" href="#no-require-exports-or-moduleexports" id="no-require-exports-or-moduleexports">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_require_exports_or_module_exports"></a></h5>
<p>In most cases, the ES module <code>import</code> can be used to load CommonJS modules.</p>
<p>If needed, a <code>require</code> function can be constructed within an ES module using
<a href="module.html#modulecreaterequirefilename"><code>module.createRequire()</code></a>.</p>
<h5>No <code>__filename</code> or <code>__dirname</code><span><a class="mark" href="#no-__filename-or-__dirname" id="no-__filename-or-__dirname">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_filename_or_dirname"></a></h5>
<p>These CommonJS variables are not available in ES modules.</p>
<p><code>__filename</code> and <code>__dirname</code> use cases can be replicated via
<a href="#importmetaurl"><code>import.meta.url</code></a>.</p>
<h5>No Native Module Loading<span><a class="mark" href="#no-native-module-loading" id="no-native-module-loading">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_native_module_loading"></a></h5>
<p>Native modules are not currently supported with ES module imports.</p>
<p>They can instead be loaded with <a href="module.html#modulecreaterequirefilename"><code>module.createRequire()</code></a> or
<a href="process.html#processdlopenmodule-filename-flags"><code>process.dlopen</code></a>.</p>
<h5>No <code>require.resolve</code><span><a class="mark" href="#no-requireresolve" id="no-requireresolve">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_require_resolve"></a></h5>
<p>Relative resolution can be handled via <code>new URL('./local', import.meta.url)</code>.</p>
<p>For a complete <code>require.resolve</code> replacement, there is a flagged experimental
<a href="#importmetaresolvespecifier-parent"><code>import.meta.resolve</code></a> API.</p>
<p>Alternatively <code>module.createRequire()</code> can be used.</p>
<h5>No <code>NODE_PATH</code><span><a class="mark" href="#no-node_path" id="no-node_path">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_node_path"></a></h5>
<p><code>NODE_PATH</code> is not part of resolving <code>import</code> specifiers. Please use symlinks
if this behavior is desired.</p>
<h5>No <code>require.extensions</code><span><a class="mark" href="#no-requireextensions" id="no-requireextensions">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_require_extensions"></a></h5>
<p><code>require.extensions</code> is not used by <code>import</code>. The expectation is that loader
hooks can provide this workflow in the future.</p>
<h5>No <code>require.cache</code><span><a class="mark" href="#no-requirecache" id="no-requirecache">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_require_cache"></a></h5>
<p><code>require.cache</code> is not used by <code>import</code> as the ES module loader has its own
separate cache.</p>
<p><i id="esm_experimental_json_modules"></i></p>
</section><section><h3>JSON modules<span><a class="mark" href="#json-modules" id="json-modules">#</a></span><a aria-hidden="true" class="legacy" id="esm_json_modules"></a></h3>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<p>JSON files can be referenced by <code>import</code>:</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> packageConfig <span class="hljs-keyword">from</span> <span class="hljs-string">'./package.json'</span> assert { <span class="hljs-attr">type</span>: <span class="hljs-string">'json'</span> };</code></pre>
<p>The <code>assert { type: 'json' }</code> syntax is mandatory; see <a href="#import-assertions">Import Assertions</a>.</p>
<p>The imported JSON only exposes a <code>default</code> export. There is no support for named
exports. A cache entry is created in the CommonJS cache to avoid duplication.
The same object is returned in CommonJS if the JSON module has already been
imported from the same path.</p>
<p><i id="esm_experimental_wasm_modules"></i></p>
</section><section><h3>Wasm modules<span><a class="mark" href="#wasm-modules" id="wasm-modules">#</a></span><a aria-hidden="true" class="legacy" id="esm_wasm_modules"></a></h3>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<p>Importing WebAssembly modules is supported under the
<code>--experimental-wasm-modules</code> flag, allowing any <code>.wasm</code> files to be
imported as normal modules while also supporting their module imports.</p>
<p>This integration is in line with the
<a href="https://github.com/webassembly/esm-integration">ES Module Integration Proposal for WebAssembly</a>.</p>
<p>For example, an <code>index.mjs</code> containing:</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> * <span class="hljs-keyword">as</span> M <span class="hljs-keyword">from</span> <span class="hljs-string">'./module.wasm'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(M);</code></pre>
<p>executed under:</p>
<pre><code class="language-bash">node --experimental-wasm-modules index.mjs</code></pre>
<p>would provide the exports interface for the instantiation of <code>module.wasm</code>.</p>
<p><i id="esm_experimental_top_level_await"></i></p>
</section><section><h3>Top-level <code>await</code><span><a class="mark" href="#top-level-await" id="top-level-await">#</a></span><a aria-hidden="true" class="legacy" id="esm_top_level_await"></a></h3>
<div class="api_metadata">
<span>Added in: v14.8.0</span>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<p>The <code>await</code> keyword may be used in the top level body of an ECMAScript module.</p>
<p>Assuming an <code>a.mjs</code> with</p>
<pre><code class="language-js"><span class="hljs-keyword">export</span> <span class="hljs-keyword">const</span> five = <span class="hljs-keyword">await</span> <span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">5</span>);</code></pre>
<p>And a <code>b.mjs</code> with</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> { five } <span class="hljs-keyword">from</span> <span class="hljs-string">'./a.mjs'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(five); <span class="hljs-comment">// Logs `5`</span></code></pre>
<pre><code class="language-bash">node b.mjs <span class="hljs-comment"># works</span></code></pre>
<p>If a top level <code>await</code> expression never resolves, the <code>node</code> process will exit
with a <code>13</code> <a href="process.html#exit-codes">status code</a>.</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> { spawn } <span class="hljs-keyword">from</span> <span class="hljs-string">'child_process'</span>;
<span class="hljs-keyword">import</span> { execPath } <span class="hljs-keyword">from</span> <span class="hljs-string">'process'</span>;
<span class="hljs-title function_">spawn</span>(execPath, [
<span class="hljs-string">'--input-type=module'</span>,
<span class="hljs-string">'--eval'</span>,
<span class="hljs-comment">// Never-resolving Promise:</span>
<span class="hljs-string">'await new Promise(() => {})'</span>,
]).<span class="hljs-title function_">once</span>(<span class="hljs-string">'exit'</span>, <span class="hljs-function">(<span class="hljs-params">code</span>) =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(code); <span class="hljs-comment">// Logs `13`</span>
});</code></pre>
</section><section><h3>HTTPS and HTTP imports<span><a class="mark" href="#https-and-http-imports" id="https-and-http-imports">#</a></span><a aria-hidden="true" class="legacy" id="esm_https_and_http_imports"></a></h3>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<p>Importing network based modules using <code>https:</code> and <code>http:</code> is supported under
the <code>--experimental-network-imports</code> flag. This allows web browser-like imports
to work in Node.js with a few differences due to application stability and
security concerns that are different when running in a privileged environment
instead of a browser sandbox.</p>
<h4>Imports are limited to HTTP/1<span><a class="mark" href="#imports-are-limited-to-http1" id="imports-are-limited-to-http1">#</a></span><a aria-hidden="true" class="legacy" id="esm_imports_are_limited_to_http_1"></a></h4>
<p>Automatic protocol negotiation for HTTP/2 and HTTP/3 is not yet supported.</p>
<h4>HTTP is limited to loopback addresses<span><a class="mark" href="#http-is-limited-to-loopback-addresses" id="http-is-limited-to-loopback-addresses">#</a></span><a aria-hidden="true" class="legacy" id="esm_http_is_limited_to_loopback_addresses"></a></h4>
<p><code>http:</code> is vulnerable to man-in-the-middle attacks and is not allowed to be
used for addresses outside of the IPv4 address <code>127.0.0.0/8</code> (<code>127.0.0.1</code> to
<code>127.255.255.255</code>) and the IPv6 address <code>::1</code>. Support for <code>http:</code> is intended
to be used for local development.</p>
<h4>Authentication is never sent to the destination server.<span><a class="mark" href="#authentication-is-never-sent-to-the-destination-server" id="authentication-is-never-sent-to-the-destination-server">#</a></span><a aria-hidden="true" class="legacy" id="esm_authentication_is_never_sent_to_the_destination_server"></a></h4>
<p><code>Authorization</code>, <code>Cookie</code>, and <code>Proxy-Authorization</code> headers are not sent to the
server. Avoid including user info in parts of imported URLs. A security model
for safely using these on the server is being worked on.</p>
<h4>CORS is never checked on the destination server<span><a class="mark" href="#cors-is-never-checked-on-the-destination-server" id="cors-is-never-checked-on-the-destination-server">#</a></span><a aria-hidden="true" class="legacy" id="esm_cors_is_never_checked_on_the_destination_server"></a></h4>
<p>CORS is designed to allow a server to limit the consumers of an API to a
specific set of hosts. This is not supported as it does not make sense for a
server-based implementation.</p>
<h4>Cannot load non-network dependencies<span><a class="mark" href="#cannot-load-non-network-dependencies" id="cannot-load-non-network-dependencies">#</a></span><a aria-hidden="true" class="legacy" id="esm_cannot_load_non_network_dependencies"></a></h4>
<p>These modules cannot access other modules that are not over <code>http:</code> or <code>https:</code>.
To still access local modules while avoiding the security concern, pass in
references to the local dependencies:</p>
<pre><code class="language-js mjs"><span class="hljs-comment">// file.mjs</span>
<span class="hljs-keyword">import</span> worker_threads <span class="hljs-keyword">from</span> <span class="hljs-string">'worker_threads'</span>;
<span class="hljs-keyword">import</span> { configure, resize } <span class="hljs-keyword">from</span> <span class="hljs-string">'https://example.com/imagelib.mjs'</span>;
<span class="hljs-title function_">configure</span>({ worker_threads });</code></pre>
<pre><code class="language-js mjs"><span class="hljs-comment">// https://example.com/imagelib.mjs</span>
<span class="hljs-keyword">let</span> worker_threads;
<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">configure</span>(<span class="hljs-params">opts</span>) {
worker_threads = opts.<span class="hljs-property">worker_threads</span>;
}
<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">resize</span>(<span class="hljs-params">img, size</span>) {
<span class="hljs-comment">// Perform resizing in worker_thread to avoid main thread blocking</span>
}</code></pre>
<h4>Network-based loading is not enabled by default<span><a class="mark" href="#network-based-loading-is-not-enabled-by-default" id="network-based-loading-is-not-enabled-by-default">#</a></span><a aria-hidden="true" class="legacy" id="esm_network_based_loading_is_not_enabled_by_default"></a></h4>
<p>For now, the <code>--experimental-network-imports</code> flag is required to enable loading
resources over <code>http:</code> or <code>https:</code>. In the future, a different mechanism will be
used to enforce this. Opt-in is required to prevent transitive dependencies
inadvertently using potentially mutable state that could affect reliability
of Node.js applications.</p>
<p><i id="esm_experimental_loaders"></i></p>
</section><section><h3>Loaders<span><a class="mark" href="#loaders" id="loaders">#</a></span><a aria-hidden="true" class="legacy" id="esm_loaders"></a></h3>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<blockquote>
<p>This API is currently being redesigned and will still change.</p>
</blockquote>
<p>To customize the default module resolution, loader hooks can optionally be
provided via a <code>--experimental-loader ./loader-name.mjs</code> argument to Node.js.</p>
<p>When hooks are used they apply to the entry point and all <code>import</code> calls. They
won't apply to <code>require</code> calls; those still follow <a href="modules.html">CommonJS</a> rules.</p>
<h4>Hooks<span><a class="mark" href="#hooks" id="hooks">#</a></span><a aria-hidden="true" class="legacy" id="esm_hooks"></a></h4>
<h5><code>resolve(specifier, context, defaultResolve)</code><span><a class="mark" href="#resolvespecifier-context-defaultresolve" id="resolvespecifier-context-defaultresolve">#</a></span><a aria-hidden="true" class="legacy" id="esm_resolve_specifier_context_defaultresolve"></a></h5>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v16.14.0</td>
<td><p>Add support for import assertions.</p></td></tr>
</tbody></table>
</details>
</div>
<blockquote>
<p>The loaders API is being redesigned. This hook may disappear or its
signature may change. Do not rely on the API described below.</p>
</blockquote>
<ul>
<li><code>specifier</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a>
<ul>
<li><code>conditions</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string[]></a></li>
<li><code>importAssertions</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li>
<li><code>parentURL</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a></li>
</ul>
</li>
<li><code>defaultResolve</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The Node.js default resolver.</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a>
<ul>
<li><code>format</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Null_type" class="type"><null></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a>
<code>'builtin' | 'commonjs' | 'json' | 'module' | 'wasm'</code></li>
<li><code>url</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The absolute url to the import target (such as <code>file://…</code>)</li>
</ul>
</li>
</ul>
<p>The <code>resolve</code> hook returns the resolved file URL for a given module specifier
and parent URL, and optionally its format (such as <code>'module'</code>) as a hint to the
<code>load</code> hook. If a format is specified, the <code>load</code> hook is ultimately responsible
for providing the final <code>format</code> value (and it is free to ignore the hint
provided by <code>resolve</code>); if <code>resolve</code> provides a <code>format</code>, a custom <code>load</code>
hook is required even if only to pass the value to the Node.js default <code>load</code>
hook.</p>
<p>The module specifier is the string in an <code>import</code> statement or
<code>import()</code> expression, and the parent URL is the URL of the module that imported
this one, or <code>undefined</code> if this is the main entry point for the application.</p>
<p>The <code>conditions</code> property in <code>context</code> is an array of conditions for
<a href="packages.html#conditional-exports">package exports conditions</a> that apply to this resolution
request. They can be used for looking up conditional mappings elsewhere or to
modify the list when calling the default resolution logic.</p>
<p>The current <a href="packages.html#conditional-exports">package exports conditions</a> are always in
the <code>context.conditions</code> array passed into the hook. To guarantee <em>default
Node.js module specifier resolution behavior</em> when calling <code>defaultResolve</code>, the
<code>context.conditions</code> array passed to it <em>must</em> include <em>all</em> elements of the
<code>context.conditions</code> array originally passed into the <code>resolve</code> hook.</p>
<pre><code class="language-js"><span class="hljs-comment">/**
* <span class="hljs-doctag">@param</span> {<span class="hljs-type">string</span>} <span class="hljs-variable">specifier</span>
* <span class="hljs-doctag">@param</span> {<span class="hljs-type">{
* conditions: string[],
* parentURL: string | undefined,
* </span>}} context
* <span class="hljs-doctag">@param</span> {<span class="hljs-type">Function</span>} <span class="hljs-variable">defaultResolve</span>
* <span class="hljs-doctag">@returns</span> {<span class="hljs-type">Promise<{ url: string </span>}>}
*/</span>
<span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">resolve</span>(<span class="hljs-params">specifier, context, defaultResolve</span>) {
<span class="hljs-keyword">const</span> { parentURL = <span class="hljs-literal">null</span> } = context;
<span class="hljs-keyword">if</span> (<span class="hljs-title class_">Math</span>.<span class="hljs-title function_">random</span>() > <span class="hljs-number">0.5</span>) { <span class="hljs-comment">// Some condition.</span>
<span class="hljs-comment">// For some or all specifiers, do some custom logic for resolving.</span>
<span class="hljs-comment">// Always return an object of the form {url: <string>}.</span>
<span class="hljs-keyword">return</span> {
<span class="hljs-attr">url</span>: parentURL ?
<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(specifier, parentURL).<span class="hljs-property">href</span> :
<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(specifier).<span class="hljs-property">href</span>,
};
}
<span class="hljs-keyword">if</span> (<span class="hljs-title class_">Math</span>.<span class="hljs-title function_">random</span>() < <span class="hljs-number">0.5</span>) { <span class="hljs-comment">// Another condition.</span>
<span class="hljs-comment">// When calling `defaultResolve`, the arguments can be modified. In this</span>
<span class="hljs-comment">// case it's adding another value for matching conditional exports.</span>
<span class="hljs-keyword">return</span> <span class="hljs-title function_">defaultResolve</span>(specifier, {
...context,
<span class="hljs-attr">conditions</span>: [...context.<span class="hljs-property">conditions</span>, <span class="hljs-string">'another-condition'</span>],
});
}
<span class="hljs-comment">// Defer to Node.js for all other specifiers.</span>
<span class="hljs-keyword">return</span> <span class="hljs-title function_">defaultResolve</span>(specifier, context, defaultResolve);
}</code></pre>
<h5><code>load(url, context, defaultLoad)</code><span><a class="mark" href="#loadurl-context-defaultload" id="loadurl-context-defaultload">#</a></span><a aria-hidden="true" class="legacy" id="esm_load_url_context_defaultload"></a></h5>
<blockquote>
<p>The loaders API is being redesigned. This hook may disappear or its
signature may change. Do not rely on the API described below.</p>
</blockquote>
<blockquote>
<p>In a previous version of this API, this was split across 3 separate, now
deprecated, hooks (<code>getFormat</code>, <code>getSource</code>, and <code>transformSource</code>).</p>
</blockquote>
<ul>
<li><code>url</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a>
<ul>
<li><code>format</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Null_type" class="type"><null></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> The format optionally supplied by the
<code>resolve</code> hook.</li>
<li><code>importAssertions</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li>
</ul>
</li>
<li><code>defaultLoad</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a>
<ul>
<li><code>format</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
<li><code>source</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer" class="type"><ArrayBuffer></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray" class="type"><TypedArray></a></li>
</ul>
</li>
</ul>
<p>The <code>load</code> hook provides a way to define a custom method of determining how
a URL should be interpreted, retrieved, and parsed. It is also in charge of
validating the import assertion.</p>
<p>The final value of <code>format</code> must be one of the following:</p>
<table><thead><tr><th><code>format</code></th><th>Description</th><th>Acceptable types for <code>source</code> returned by <code>load</code></th></tr></thead><tbody><tr><td><code>'builtin'</code></td><td>Load a Node.js builtin module</td><td>Not applicable</td></tr><tr><td><code>'commonjs'</code></td><td>Load a Node.js CommonJS module</td><td>Not applicable</td></tr><tr><td><code>'json'</code></td><td>Load a JSON file</td><td>{ <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String"><code>string</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray"><code>TypedArray</code></a> }</td></tr><tr><td><code>'module'</code></td><td>Load an ES module</td><td>{ <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String"><code>string</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray"><code>TypedArray</code></a> }</td></tr><tr><td><code>'wasm'</code></td><td>Load a WebAssembly module</td><td>{ <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray"><code>TypedArray</code></a> }</td></tr></tbody></table>
<p>The value of <code>source</code> is ignored for type <code>'builtin'</code> because currently it is
not possible to replace the value of a Node.js builtin (core) module. The value
of <code>source</code> is ignored for type <code>'commonjs'</code> because the CommonJS module loader
does not provide a mechanism for the ES module loader to override the
<a href="#commonjs-namespaces">CommonJS module return value</a>. This limitation might be
overcome in the future.</p>
<blockquote>
<p><strong>Caveat</strong>: The ESM <code>load</code> hook and namespaced exports from CommonJS modules
are incompatible. Attempting to use them together will result in an empty
object from the import. This may be addressed in the future.</p>
</blockquote>
<blockquote>
<p>These types all correspond to classes defined in ECMAScript.</p>
</blockquote>
<ul>
<li>The specific <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a> object is a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer"><code>SharedArrayBuffer</code></a>.</li>
<li>The specific <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray"><code>TypedArray</code></a> object is a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array"><code>Uint8Array</code></a>.</li>
</ul>
<p>If the source value of a text-based format (i.e., <code>'json'</code>, <code>'module'</code>)
is not a string, it is converted to a string using <a href="util.html#class-utiltextdecoder"><code>util.TextDecoder</code></a>.</p>
<p>The <code>load</code> hook provides a way to define a custom method for retrieving the
source code of an ES module specifier. This would allow a loader to potentially
avoid reading files from disk. It could also be used to map an unrecognized
format to a supported one, for example <code>yaml</code> to <code>module</code>.</p>
<pre><code class="language-js"><span class="hljs-comment">/**
* <span class="hljs-doctag">@param</span> {<span class="hljs-type">string</span>} <span class="hljs-variable">url</span>
* <span class="hljs-doctag">@param</span> {<span class="hljs-type">{
format: string,
</span>}} context If resolve settled with a `format`, that value is included here.
* <span class="hljs-doctag">@param</span> {<span class="hljs-type">Function</span>} <span class="hljs-variable">defaultLoad</span>
* <span class="hljs-doctag">@returns</span> {<span class="hljs-type">Promise<{
format: string,
source: string | ArrayBuffer | SharedArrayBuffer | Uint8Array,
</span>}>}
*/</span>
<span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">load</span>(<span class="hljs-params">url, context, defaultLoad</span>) {
<span class="hljs-keyword">const</span> { format } = context;
<span class="hljs-keyword">if</span> (<span class="hljs-title class_">Math</span>.<span class="hljs-title function_">random</span>() > <span class="hljs-number">0.5</span>) { <span class="hljs-comment">// Some condition.</span>
<span class="hljs-comment">/*
For some or all URLs, do some custom logic for retrieving the source.
Always return an object of the form {
format: <string>,
source: <string|buffer>,
}.
*/</span>
<span class="hljs-keyword">return</span> {
format,
<span class="hljs-attr">source</span>: <span class="hljs-string">'...'</span>,
};
}
<span class="hljs-comment">// Defer to Node.js for all other URLs.</span>
<span class="hljs-keyword">return</span> <span class="hljs-title function_">defaultLoad</span>(url, context, defaultLoad);
}</code></pre>
<p>In a more advanced scenario, this can also be used to transform an unsupported
source to a supported one (see <a href="#examples">Examples</a> below).</p>
<h5><code>globalPreload()</code><span><a class="mark" href="#globalpreload" id="globalpreload">#</a></span><a aria-hidden="true" class="legacy" id="esm_globalpreload"></a></h5>
<blockquote>
<p>The loaders API is being redesigned. This hook may disappear or its
signature may change. Do not rely on the API described below.</p>
</blockquote>
<blockquote>
<p>In a previous version of this API, this hook was named
<code>getGlobalPreloadCode</code>.</p>
</blockquote>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
</ul>
<p>Sometimes it might be necessary to run some code inside of the same global
scope that the application runs in. This hook allows the return of a string
that is run as a sloppy-mode script on startup.</p>
<p>Similar to how CommonJS wrappers work, the code runs in an implicit function
scope. The only argument is a <code>require</code>-like function that can be used to load
builtins like "fs": <code>getBuiltin(request: string)</code>.</p>
<p>If the code needs more advanced <code>require</code> features, it has to construct
its own <code>require</code> using <code>module.createRequire()</code>.</p>
<pre><code class="language-js"><span class="hljs-comment">/**
* <span class="hljs-doctag">@param</span> {<span class="hljs-type">{
port: MessagePort,
</span>}} utilities Things that preload code might find useful
* <span class="hljs-doctag">@returns</span> {<span class="hljs-type">string</span>} Code to run before application startup
*/</span>
<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">globalPreload</span>(<span class="hljs-params">utilities</span>) {
<span class="hljs-keyword">return</span> <span class="hljs-string">`\
globalThis.someInjectedProperty = 42;
console.log('I just set some globals!');
const { createRequire } = getBuiltin('module');
const { cwd } = getBuiltin('process');
const require = createRequire(cwd() + '/<preload>');
// [...]
`</span>;
}</code></pre>
<p>In order to allow communication between the application and the loader, another
argument is provided to the preload code: <code>port</code>. This is available as a
parameter to the loader hook and inside of the source text returned by the hook.
Some care must be taken in order to properly call <a href="https://nodejs.org/dist/latest-v17.x/docs/api/worker_threads.html#portref"><code>port.ref()</code></a> and
<a href="https://nodejs.org/dist/latest-v17.x/docs/api/worker_threads.html#portunref"><code>port.unref()</code></a> to prevent a process from being in a state where it won't
close normally.</p>
<pre><code class="language-js"><span class="hljs-comment">/**
* This example has the application context send a message to the loader
* and sends the message back to the application context
* <span class="hljs-doctag">@param</span> {<span class="hljs-type">{
port: MessagePort,
</span>}} utilities Things that preload code might find useful
* <span class="hljs-doctag">@returns</span> {<span class="hljs-type">string</span>} Code to run before application startup
*/</span>
<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">globalPreload</span>(<span class="hljs-params">{ port }</span>) {
port.<span class="hljs-property">onmessage</span> = <span class="hljs-function">(<span class="hljs-params">evt</span>) =></span> {
port.<span class="hljs-title function_">postMessage</span>(evt.<span class="hljs-property">data</span>);
};
<span class="hljs-keyword">return</span> <span class="hljs-string">`\
port.postMessage('console.log("I went to the Loader and back");');
port.onmessage = (evt) => {
eval(evt.data);
};
`</span>;
}</code></pre>
<h4>Examples<span><a class="mark" href="#examples" id="examples">#</a></span><a aria-hidden="true" class="legacy" id="esm_examples"></a></h4>
<p>The various loader hooks can be used together to accomplish wide-ranging
customizations of Node.js’ code loading and evaluation behaviors.</p>
<h5>HTTPS loader<span><a class="mark" href="#https-loader" id="https-loader">#</a></span><a aria-hidden="true" class="legacy" id="esm_https_loader"></a></h5>
<p>In current Node.js, specifiers starting with <code>https://</code> are unsupported. The
loader below registers hooks to enable rudimentary support for such specifiers.
While this may seem like a significant improvement to Node.js core
functionality, there are substantial downsides to actually using this loader:
performance is much slower than loading files from disk, there is no caching,
and there is no security.</p>
<pre><code class="language-js"><span class="hljs-comment">// https-loader.mjs</span>
<span class="hljs-keyword">import</span> { get } <span class="hljs-keyword">from</span> <span class="hljs-string">'https'</span>;
<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">resolve</span>(<span class="hljs-params">specifier, context, defaultResolve</span>) {
<span class="hljs-keyword">const</span> { parentURL = <span class="hljs-literal">null</span> } = context;
<span class="hljs-comment">// Normally Node.js would error on specifiers starting with 'https://', so</span>
<span class="hljs-comment">// this hook intercepts them and converts them into absolute URLs to be</span>
<span class="hljs-comment">// passed along to the later hooks below.</span>
<span class="hljs-keyword">if</span> (specifier.<span class="hljs-title function_">startsWith</span>(<span class="hljs-string">'https://'</span>)) {
<span class="hljs-keyword">return</span> {
<span class="hljs-attr">url</span>: specifier
};
} <span class="hljs-keyword">else</span> <span class="hljs-keyword">if</span> (parentURL && parentURL.<span class="hljs-title function_">startsWith</span>(<span class="hljs-string">'https://'</span>)) {
<span class="hljs-keyword">return</span> {
<span class="hljs-attr">url</span>: <span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(specifier, parentURL).<span class="hljs-property">href</span>
};
}
<span class="hljs-comment">// Let Node.js handle all other specifiers.</span>
<span class="hljs-keyword">return</span> <span class="hljs-title function_">defaultResolve</span>(specifier, context, defaultResolve);
}
<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">load</span>(<span class="hljs-params">url, context, defaultLoad</span>) {
<span class="hljs-comment">// For JavaScript to be loaded over the network, we need to fetch and</span>
<span class="hljs-comment">// return it.</span>
<span class="hljs-keyword">if</span> (url.<span class="hljs-title function_">startsWith</span>(<span class="hljs-string">'https://'</span>)) {
<span class="hljs-keyword">return</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Promise</span>(<span class="hljs-function">(<span class="hljs-params">resolve, reject</span>) =></span> {
<span class="hljs-title function_">get</span>(url, <span class="hljs-function">(<span class="hljs-params">res</span>) =></span> {
<span class="hljs-keyword">let</span> data = <span class="hljs-string">''</span>;
res.<span class="hljs-title function_">on</span>(<span class="hljs-string">'data'</span>, <span class="hljs-function">(<span class="hljs-params">chunk</span>) =></span> data += chunk);
res.<span class="hljs-title function_">on</span>(<span class="hljs-string">'end'</span>, <span class="hljs-function">() =></span> <span class="hljs-title function_">resolve</span>({
<span class="hljs-comment">// This example assumes all network-provided JavaScript is ES module</span>
<span class="hljs-comment">// code.</span>
<span class="hljs-attr">format</span>: <span class="hljs-string">'module'</span>,
<span class="hljs-attr">source</span>: data,
}));
}).<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">err</span>) =></span> <span class="hljs-title function_">reject</span>(err));
});
}
<span class="hljs-comment">// Let Node.js handle all other URLs.</span>
<span class="hljs-keyword">return</span> <span class="hljs-title function_">defaultLoad</span>(url, context, defaultLoad);
}</code></pre>
<pre><code class="language-js"><span class="hljs-comment">// main.mjs</span>
<span class="hljs-keyword">import</span> { <span class="hljs-variable constant_">VERSION</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'https://coffeescript.org/browser-compiler-modern/coffeescript.js'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-variable constant_">VERSION</span>);</code></pre>
<p>With the preceding loader, running
<code>node --experimental-loader ./https-loader.mjs ./main.mjs</code>
prints the current version of CoffeeScript per the module at the URL in
<code>main.mjs</code>.</p>
<h5>Transpiler loader<span><a class="mark" href="#transpiler-loader" id="transpiler-loader">#</a></span><a aria-hidden="true" class="legacy" id="esm_transpiler_loader"></a></h5>
<p>Sources that are in formats Node.js doesn’t understand can be converted into
JavaScript using the <a href="#loadurl-context-defaultload"><code>load</code> hook</a>. Before that hook gets called,
however, a <a href="#resolvespecifier-context-defaultresolve"><code>resolve</code> hook</a> needs to tell Node.js not to
throw an error on unknown file types.</p>
<p>This is less performant than transpiling source files before running
Node.js; a transpiler loader should only be used for development and testing
purposes.</p>
<pre><code class="language-js"><span class="hljs-comment">// coffeescript-loader.mjs</span>
<span class="hljs-keyword">import</span> { readFile } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs/promises'</span>;
<span class="hljs-keyword">import</span> { dirname, extname, resolve <span class="hljs-keyword">as</span> resolvePath } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:path'</span>;
<span class="hljs-keyword">import</span> { cwd } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span>;
<span class="hljs-keyword">import</span> { fileURLToPath, pathToFileURL } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:url'</span>;
<span class="hljs-keyword">import</span> <span class="hljs-title class_">CoffeeScript</span> <span class="hljs-keyword">from</span> <span class="hljs-string">'coffeescript'</span>;
<span class="hljs-keyword">const</span> baseURL = <span class="hljs-title function_">pathToFileURL</span>(<span class="hljs-string">`<span class="hljs-subst">${cwd()}</span>/`</span>).<span class="hljs-property">href</span>;
<span class="hljs-comment">// CoffeeScript files end in .coffee, .litcoffee or .coffee.md.</span>
<span class="hljs-keyword">const</span> extensionsRegex = <span class="hljs-regexp">/\.coffee$|\.litcoffee$|\.coffee\.md$/</span>;
<span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">resolve</span>(<span class="hljs-params">specifier, context, defaultResolve</span>) {
<span class="hljs-keyword">const</span> { parentURL = baseURL } = context;
<span class="hljs-comment">// Node.js normally errors on unknown file extensions, so return a URL for</span>
<span class="hljs-comment">// specifiers ending in the CoffeeScript file extensions.</span>
<span class="hljs-keyword">if</span> (extensionsRegex.<span class="hljs-title function_">test</span>(specifier)) {
<span class="hljs-keyword">return</span> {
<span class="hljs-attr">url</span>: <span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(specifier, parentURL).<span class="hljs-property">href</span>
};
}
<span class="hljs-comment">// Let Node.js handle all other specifiers.</span>
<span class="hljs-keyword">return</span> <span class="hljs-title function_">defaultResolve</span>(specifier, context, defaultResolve);
}
<span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">load</span>(<span class="hljs-params">url, context, defaultLoad</span>) {
<span class="hljs-comment">// Now that we patched resolve to let CoffeeScript URLs through, we need to</span>
<span class="hljs-comment">// tell Node.js what format such URLs should be interpreted as. Because</span>
<span class="hljs-comment">// CoffeeScript transpiles into JavaScript, it should be one of the two</span>
<span class="hljs-comment">// JavaScript formats: 'commonjs' or 'module'.</span>
<span class="hljs-keyword">if</span> (extensionsRegex.<span class="hljs-title function_">test</span>(url)) {
<span class="hljs-comment">// CoffeeScript files can be either CommonJS or ES modules, so we want any</span>
<span class="hljs-comment">// CoffeeScript file to be treated by Node.js the same as a .js file at the</span>
<span class="hljs-comment">// same location. To determine how Node.js would interpret an arbitrary .js</span>
<span class="hljs-comment">// file, search up the file system for the nearest parent package.json file</span>
<span class="hljs-comment">// and read its "type" field.</span>
<span class="hljs-keyword">const</span> format = <span class="hljs-keyword">await</span> <span class="hljs-title function_">getPackageType</span>(url);
<span class="hljs-comment">// When a hook returns a format of 'commonjs', `source` is be ignored.</span>
<span class="hljs-comment">// To handle CommonJS files, a handler needs to be registered with</span>
<span class="hljs-comment">// `require.extensions` in order to process the files with the CommonJS</span>
<span class="hljs-comment">// loader. Avoiding the need for a separate CommonJS handler is a future</span>
<span class="hljs-comment">// enhancement planned for ES module loaders.</span>
<span class="hljs-keyword">if</span> (format === <span class="hljs-string">'commonjs'</span>) {
<span class="hljs-keyword">return</span> { format };
}
<span class="hljs-keyword">const</span> { <span class="hljs-attr">source</span>: rawSource } = <span class="hljs-keyword">await</span> <span class="hljs-title function_">defaultLoad</span>(url, { format });
<span class="hljs-comment">// This hook converts CoffeeScript source code into JavaScript source code</span>
<span class="hljs-comment">// for all imported CoffeeScript files.</span>
<span class="hljs-keyword">const</span> transformedSource = <span class="hljs-title class_">CoffeeScript</span>.<span class="hljs-title function_">compile</span>(rawSource.<span class="hljs-title function_">toString</span>(), {
<span class="hljs-attr">bare</span>: <span class="hljs-literal">true</span>,
<span class="hljs-attr">filename</span>: url,
});
<span class="hljs-keyword">return</span> {
format,
<span class="hljs-attr">source</span>: transformedSource,
};
}
<span class="hljs-comment">// Let Node.js handle all other URLs.</span>
<span class="hljs-keyword">return</span> <span class="hljs-title function_">defaultLoad</span>(url, context, defaultLoad);
}
<span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">getPackageType</span>(<span class="hljs-params">url</span>) {
<span class="hljs-comment">// `url` is only a file path during the first iteration when passed the</span>
<span class="hljs-comment">// resolved url from the load() hook</span>
<span class="hljs-comment">// an actual file path from load() will contain a file extension as it's</span>
<span class="hljs-comment">// required by the spec</span>
<span class="hljs-comment">// this simple truthy check for whether `url` contains a file extension will</span>
<span class="hljs-comment">// work for most projects but does not cover some edge-cases (such as</span>
<span class="hljs-comment">// extensionless files or a url ending in a trailing space)</span>
<span class="hljs-keyword">const</span> isFilePath = !!<span class="hljs-title function_">extname</span>(url);
<span class="hljs-comment">// If it is a file path, get the directory it's in</span>
<span class="hljs-keyword">const</span> dir = isFilePath ?
<span class="hljs-title function_">dirname</span>(<span class="hljs-title function_">fileURLToPath</span>(url)) :
url;
<span class="hljs-comment">// Compose a file path to a package.json in the same directory,</span>
<span class="hljs-comment">// which may or may not exist</span>
<span class="hljs-keyword">const</span> packagePath = <span class="hljs-title function_">resolvePath</span>(dir, <span class="hljs-string">'package.json'</span>);
<span class="hljs-comment">// Try to read the possibly nonexistent package.json</span>
<span class="hljs-keyword">const</span> type = <span class="hljs-keyword">await</span> <span class="hljs-title function_">readFile</span>(packagePath, { <span class="hljs-attr">encoding</span>: <span class="hljs-string">'utf8'</span> })
.<span class="hljs-title function_">then</span>(<span class="hljs-function">(<span class="hljs-params">filestring</span>) =></span> <span class="hljs-title class_">JSON</span>.<span class="hljs-title function_">parse</span>(filestring).<span class="hljs-property">type</span>)
.<span class="hljs-title function_">catch</span>(<span class="hljs-function">(<span class="hljs-params">err</span>) =></span> {
<span class="hljs-keyword">if</span> (err?.<span class="hljs-property">code</span> !== <span class="hljs-string">'ENOENT'</span>) <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(err);
});
<span class="hljs-comment">// Ff package.json existed and contained a `type` field with a value, voila</span>
<span class="hljs-keyword">if</span> (type) <span class="hljs-keyword">return</span> type;
<span class="hljs-comment">// Otherwise, (if not at the root) continue checking the next directory up</span>
<span class="hljs-comment">// If at the root, stop and return false</span>
<span class="hljs-keyword">return</span> dir.<span class="hljs-property">length</span> > <span class="hljs-number">1</span> && <span class="hljs-title function_">getPackageType</span>(<span class="hljs-title function_">resolvePath</span>(dir, <span class="hljs-string">'..'</span>));
}</code></pre>
<pre><code class="language-coffee"><span class="hljs-comment"># main.coffee</span>
<span class="hljs-keyword">import</span> { scream } <span class="hljs-keyword">from</span> <span class="hljs-string">'./scream.coffee'</span>
console.log scream <span class="hljs-string">'hello, world'</span>
<span class="hljs-keyword">import</span> { version } <span class="hljs-keyword">from</span> <span class="hljs-string">'process'</span>
console.log <span class="hljs-string">"Brought to you by Node.js version <span class="hljs-subst">#{version}</span>"</span></code></pre>
<pre><code class="language-coffee"><span class="hljs-comment"># scream.coffee</span>
<span class="hljs-keyword">export</span> scream = <span class="hljs-function"><span class="hljs-params">(str)</span> -></span> str.toUpperCase()</code></pre>
<p>With the preceding loader, running
<code>node --experimental-loader ./coffeescript-loader.mjs main.coffee</code>
causes <code>main.coffee</code> to be turned into JavaScript after its source code is
loaded from disk but before Node.js executes it; and so on for any <code>.coffee</code>,
<code>.litcoffee</code> or <code>.coffee.md</code> files referenced via <code>import</code> statements of any
loaded file.</p>
</section><section><h3>Resolution algorithm<span><a class="mark" href="#resolution-algorithm" id="resolution-algorithm">#</a></span><a aria-hidden="true" class="legacy" id="esm_resolution_algorithm"></a></h3>
<h4>Features<span><a class="mark" href="#features" id="features">#</a></span><a aria-hidden="true" class="legacy" id="esm_features"></a></h4>
<p>The resolver has the following properties:</p>
<ul>
<li>FileURL-based resolution as is used by ES modules</li>
<li>Support for builtin module loading</li>
<li>Relative and absolute URL resolution</li>
<li>No default extensions</li>
<li>No folder mains</li>
<li>Bare specifier package resolution lookup through node_modules</li>
</ul>
<h4>Resolver algorithm<span><a class="mark" href="#resolver-algorithm" id="resolver-algorithm">#</a></span><a aria-hidden="true" class="legacy" id="esm_resolver_algorithm"></a></h4>
<p>The algorithm to load an ES module specifier is given through the
<strong>ESM_RESOLVE</strong> method below. It returns the resolved URL for a
module specifier relative to a parentURL.</p>
<p>The algorithm to determine the module format of a resolved URL is
provided by <strong>ESM_FORMAT</strong>, which returns the unique module
format for any file. The <em>"module"</em> format is returned for an ECMAScript
Module, while the <em>"commonjs"</em> format is used to indicate loading through the
legacy CommonJS loader. Additional formats such as <em>"addon"</em> can be extended in
future updates.</p>
<p>In the following algorithms, all subroutine errors are propagated as errors
of these top-level routines unless stated otherwise.</p>
<p><em>defaultConditions</em> is the conditional environment name array,
<code>["node", "import"]</code>.</p>
<p>The resolver can throw the following errors:</p>
<ul>
<li><em>Invalid Module Specifier</em>: Module specifier is an invalid URL, package name
or package subpath specifier.</li>
<li><em>Invalid Package Configuration</em>: package.json configuration is invalid or
contains an invalid configuration.</li>
<li><em>Invalid Package Target</em>: Package exports or imports define a target module
for the package that is an invalid type or string target.</li>
<li><em>Package Path Not Exported</em>: Package exports do not define or permit a target
subpath in the package for the given module.</li>
<li><em>Package Import Not Defined</em>: Package imports do not define the specifier.</li>
<li><em>Module Not Found</em>: The package or module requested does not exist.</li>
<li><em>Unsupported Directory Import</em>: The resolved path corresponds to a directory,
which is not a supported target for module imports.</li>
</ul>
<h4>Resolver Algorithm Specification<span><a class="mark" href="#resolver-algorithm-specification" id="resolver-algorithm-specification">#</a></span><a aria-hidden="true" class="legacy" id="esm_resolver_algorithm_specification"></a></h4>
<p><strong>ESM_RESOLVE</strong>(<em>specifier</em>, <em>parentURL</em>)</p>
<blockquote>
<ol>
<li>Let <em>resolved</em> be <strong>undefined</strong>.</li>
<li>If <em>specifier</em> is a valid URL, then
<ol>
<li>Set <em>resolved</em> to the result of parsing and reserializing
<em>specifier</em> as a URL.</li>
</ol>
</li>
<li>Otherwise, if <em>specifier</em> starts with <em>"/"</em>, <em>"./"</em> or <em>"../"</em>, then
<ol>
<li>Set <em>resolved</em> to the URL resolution of <em>specifier</em> relative to
<em>parentURL</em>.</li>
</ol>
</li>
<li>Otherwise, if <em>specifier</em> starts with <em>"#"</em>, then
<ol>
<li>Set <em>resolved</em> to the destructured value of the result of
<strong>PACKAGE_IMPORTS_RESOLVE</strong>(<em>specifier</em>, <em>parentURL</em>,
<em>defaultConditions</em>).</li>
</ol>
</li>
<li>Otherwise,
<ol>
<li>Note: <em>specifier</em> is now a bare specifier.</li>
<li>Set <em>resolved</em> the result of
<strong>PACKAGE_RESOLVE</strong>(<em>specifier</em>, <em>parentURL</em>).</li>
</ol>
</li>
<li>Let <em>format</em> be <strong>undefined</strong>.</li>
<li>If <em>resolved</em> is a <em>"file:"</em> URL, then
<ol>
<li>If <em>resolved</em> contains any percent encodings of <em>"/"</em> or <em>"\"</em> (<em>"%2F"</em>
and <em>"%5C"</em> respectively), then
<ol>
<li>Throw an <em>Invalid Module Specifier</em> error.</li>
</ol>
</li>
<li>If the file at <em>resolved</em> is a directory, then
<ol>
<li>Throw an <em>Unsupported Directory Import</em> error.</li>
</ol>
</li>
<li>If the file at <em>resolved</em> does not exist, then
<ol>
<li>Throw a <em>Module Not Found</em> error.</li>
</ol>
</li>
<li>Set <em>resolved</em> to the real path of <em>resolved</em>, maintaining the
same URL querystring and fragment components.</li>
<li>Set <em>format</em> to the result of <strong>ESM_FILE_FORMAT</strong>(<em>resolved</em>).</li>
</ol>
</li>
<li>Otherwise,
<ol>
<li>Set <em>format</em> the module format of the content type associated with the
URL <em>resolved</em>.</li>
</ol>
</li>
<li>Load <em>resolved</em> as module format, <em>format</em>.</li>
</ol>
</blockquote>
<p><strong>PACKAGE_RESOLVE</strong>(<em>packageSpecifier</em>, <em>parentURL</em>)</p>
<blockquote>
<ol>
<li>Let <em>packageName</em> be <strong>undefined</strong>.</li>
<li>If <em>packageSpecifier</em> is an empty string, then
<ol>
<li>Throw an <em>Invalid Module Specifier</em> error.</li>
</ol>
</li>
<li>If <em>packageSpecifier</em> is a Node.js builtin module name, then
<ol>
<li>Return the string <em>"node:"</em> concatenated with <em>packageSpecifier</em>.</li>
</ol>
</li>
<li>If <em>packageSpecifier</em> does not start with <em>"@"</em>, then
<ol>
<li>Set <em>packageName</em> to the substring of <em>packageSpecifier</em> until the first
<em>"/"</em> separator or the end of the string.</li>
</ol>
</li>
<li>Otherwise,
<ol>
<li>If <em>packageSpecifier</em> does not contain a <em>"/"</em> separator, then
<ol>
<li>Throw an <em>Invalid Module Specifier</em> error.</li>
</ol>
</li>
<li>Set <em>packageName</em> to the substring of <em>packageSpecifier</em>
until the second <em>"/"</em> separator or the end of the string.</li>
</ol>
</li>
<li>If <em>packageName</em> starts with <em>"."</em> or contains <em>"\"</em> or <em>"%"</em>, then
<ol>
<li>Throw an <em>Invalid Module Specifier</em> error.</li>
</ol>
</li>
<li>Let <em>packageSubpath</em> be <em>"."</em> concatenated with the substring of
<em>packageSpecifier</em> from the position at the length of <em>packageName</em>.</li>
<li>If <em>packageSubpath</em> ends in <em>"/"</em>, then
<ol>
<li>Throw an <em>Invalid Module Specifier</em> error.</li>
</ol>
</li>
<li>Let <em>selfUrl</em> be the result of
<strong>PACKAGE_SELF_RESOLVE</strong>(<em>packageName</em>, <em>packageSubpath</em>, <em>parentURL</em>).</li>
<li>If <em>selfUrl</em> is not <strong>undefined</strong>, return <em>selfUrl</em>.</li>
<li>While <em>parentURL</em> is not the file system root,
<ol>
<li>Let <em>packageURL</em> be the URL resolution of <em>"node_modules/"</em>
concatenated with <em>packageSpecifier</em>, relative to <em>parentURL</em>.</li>
<li>Set <em>parentURL</em> to the parent folder URL of <em>parentURL</em>.</li>
<li>If the folder at <em>packageURL</em> does not exist, then
<ol>
<li>Continue the next loop iteration.</li>
</ol>
</li>
<li>Let <em>pjson</em> be the result of <strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>).</li>
<li>If <em>pjson</em> is not <strong>null</strong> and <em>pjson</em>.<em>exports</em> is not <strong>null</strong> or
<strong>undefined</strong>, then
<ol>
<li>Return the result of <strong>PACKAGE_EXPORTS_RESOLVE</strong>(<em>packageURL</em>,
<em>packageSubpath</em>, <em>pjson.exports</em>, <em>defaultConditions</em>).</li>
</ol>
</li>
<li>Otherwise, if <em>packageSubpath</em> is equal to <em>"."</em>, then
<ol>
<li>If <em>pjson.main</em> is a string, then
<ol>
<li>Return the URL resolution of <em>main</em> in <em>packageURL</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Otherwise,
<ol>
<li>Return the URL resolution of <em>packageSubpath</em> in <em>packageURL</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Throw a <em>Module Not Found</em> error.</li>
</ol>
</blockquote>
<p><strong>PACKAGE_SELF_RESOLVE</strong>(<em>packageName</em>, <em>packageSubpath</em>, <em>parentURL</em>)</p>
<blockquote>
<ol>
<li>Let <em>packageURL</em> be the result of <strong>LOOKUP_PACKAGE_SCOPE</strong>(<em>parentURL</em>).</li>
<li>If <em>packageURL</em> is <strong>null</strong>, then
<ol>
<li>Return <strong>undefined</strong>.</li>
</ol>
</li>
<li>Let <em>pjson</em> be the result of <strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>).</li>
<li>If <em>pjson</em> is <strong>null</strong> or if <em>pjson</em>.<em>exports</em> is <strong>null</strong> or
<strong>undefined</strong>, then
<ol>
<li>Return <strong>undefined</strong>.</li>
</ol>
</li>
<li>If <em>pjson.name</em> is equal to <em>packageName</em>, then
<ol>
<li>Return the <em>resolved</em> destructured value of the result of
<strong>PACKAGE_EXPORTS_RESOLVE</strong>(<em>packageURL</em>, <em>packageSubpath</em>,
<em>pjson.exports</em>, <em>defaultConditions</em>).</li>
</ol>
</li>
<li>Otherwise, return <strong>undefined</strong>.</li>
</ol>
</blockquote>
<p><strong>PACKAGE_EXPORTS_RESOLVE</strong>(<em>packageURL</em>, <em>subpath</em>, <em>exports</em>, <em>conditions</em>)</p>
<blockquote>
<ol>
<li>If <em>exports</em> is an Object with both a key starting with <em>"."</em> and a key not
starting with <em>"."</em>, throw an <em>Invalid Package Configuration</em> error.</li>
<li>If <em>subpath</em> is equal to <em>"."</em>, then
<ol>
<li>Let <em>mainExport</em> be <strong>undefined</strong>.</li>
<li>If <em>exports</em> is a String or Array, or an Object containing no keys
starting with <em>"."</em>, then
<ol>
<li>Set <em>mainExport</em> to <em>exports</em>.</li>
</ol>
</li>
<li>Otherwise if <em>exports</em> is an Object containing a <em>"."</em> property, then
<ol>
<li>Set <em>mainExport</em> to <em>exports</em>[<em>"."</em>].</li>
</ol>
</li>
<li>If <em>mainExport</em> is not <strong>undefined</strong>, then
<ol>
<li>Let <em>resolved</em> be the result of <strong>PACKAGE_TARGET_RESOLVE</strong>(
<em>packageURL</em>, <em>mainExport</em>, <em>""</em>, <strong>false</strong>, <strong>false</strong>,
<em>conditions</em>).</li>
<li>If <em>resolved</em> is not <strong>null</strong> or <strong>undefined</strong>, then
<ol>
<li>Return <em>resolved</em>.</li>
</ol>
</li>
</ol>
</li>
</ol>
</li>
<li>Otherwise, if <em>exports</em> is an Object and all keys of <em>exports</em> start with
<em>"."</em>, then
<ol>
<li>Let <em>matchKey</em> be the string <em>"./"</em> concatenated with <em>subpath</em>.</li>
<li>Let <em>resolvedMatch</em> be result of <strong>PACKAGE_IMPORTS_EXPORTS_RESOLVE</strong>(
<em>matchKey</em>, <em>exports</em>, <em>packageURL</em>, <strong>false</strong>, <em>conditions</em>).</li>
<li>If <em>resolvedMatch</em>.<em>resolve</em> is not <strong>null</strong> or <strong>undefined</strong>, then
<ol>
<li>Return <em>resolvedMatch</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Throw a <em>Package Path Not Exported</em> error.</li>
</ol>
</blockquote>
<p><strong>PACKAGE_IMPORTS_RESOLVE</strong>(<em>specifier</em>, <em>parentURL</em>, <em>conditions</em>)</p>
<blockquote>
<ol>
<li>Assert: <em>specifier</em> begins with <em>"#"</em>.</li>
<li>If <em>specifier</em> is exactly equal to <em>"#"</em> or starts with <em>"#/"</em>, then
<ol>
<li>Throw an <em>Invalid Module Specifier</em> error.</li>
</ol>
</li>
<li>Let <em>packageURL</em> be the result of <strong>LOOKUP_PACKAGE_SCOPE</strong>(<em>parentURL</em>).</li>
<li>If <em>packageURL</em> is not <strong>null</strong>, then
<ol>
<li>Let <em>pjson</em> be the result of <strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>).</li>
<li>If <em>pjson.imports</em> is a non-null Object, then
<ol>
<li>Let <em>resolvedMatch</em> be the result of
<strong>PACKAGE_IMPORTS_EXPORTS_RESOLVE</strong>(<em>specifier</em>, <em>pjson.imports</em>,
<em>packageURL</em>, <strong>true</strong>, <em>conditions</em>).</li>
<li>If <em>resolvedMatch</em>.<em>resolve</em> is not <strong>null</strong> or <strong>undefined</strong>, then
<ol>
<li>Return <em>resolvedMatch</em>.</li>
</ol>
</li>
</ol>
</li>
</ol>
</li>
<li>Throw a <em>Package Import Not Defined</em> error.</li>
</ol>
</blockquote>
<p><strong>PACKAGE_IMPORTS_EXPORTS_RESOLVE</strong>(<em>matchKey</em>, <em>matchObj</em>, <em>packageURL</em>,
<em>isImports</em>, <em>conditions</em>)</p>
<blockquote>
<ol>
<li>If <em>matchKey</em> is a key of <em>matchObj</em> and does not end in <em>"/"</em> or contain
<em>"*"</em>, then
<ol>
<li>Let <em>target</em> be the value of <em>matchObj</em>[<em>matchKey</em>].</li>
<li>Let <em>resolved</em> be the result of <strong>PACKAGE_TARGET_RESOLVE</strong>(
<em>packageURL</em>, <em>target</em>, <em>""</em>, <strong>false</strong>, <em>isImports</em>, <em>conditions</em>).</li>
<li>Return the object <em>{ resolved, exact: <strong>true</strong> }</em>.</li>
</ol>
</li>
<li>Let <em>expansionKeys</em> be the list of keys of <em>matchObj</em> either ending in
<em>"/"</em> or containing only a single <em>"*"</em>, sorted by the sorting function
<strong>PATTERN_KEY_COMPARE</strong> which orders in descending order of specificity.</li>
<li>For each key <em>expansionKey</em> in <em>expansionKeys</em>, do
<ol>
<li>Let <em>patternBase</em> be <strong>null</strong>.</li>
<li>If <em>expansionKey</em> contains <em>"*"</em>, set <em>patternBase</em> to the substring of
<em>expansionKey</em> up to but excluding the first <em>"*"</em> character.</li>
<li>If <em>patternBase</em> is not <strong>null</strong> and <em>matchKey</em> starts with but is not
equal to <em>patternBase</em>, then
<ol>
<li>If <em>matchKey</em> ends with <em>"/"</em>, throw an <em>Invalid Module Specifier</em>
error.</li>
<li>Let <em>patternTrailer</em> be the substring of <em>expansionKey</em> from the
index after the first <em>"*"</em> character.</li>
<li>If <em>patternTrailer</em> has zero length, or if <em>matchKey</em> ends with
<em>patternTrailer</em> and the length of <em>matchKey</em> is greater than or
equal to the length of <em>expansionKey</em>, then
<ol>
<li>Let <em>target</em> be the value of <em>matchObj</em>[<em>expansionKey</em>].</li>
<li>Let <em>subpath</em> be the substring of <em>matchKey</em> starting at the
index of the length of <em>patternBase</em> up to the length of
<em>matchKey</em> minus the length of <em>patternTrailer</em>.</li>
<li>Let <em>resolved</em> be the result of <strong>PACKAGE_TARGET_RESOLVE</strong>(
<em>packageURL</em>, <em>target</em>, <em>subpath</em>, <strong>true</strong>, <em>isImports</em>,
<em>conditions</em>).</li>
<li>Return the object <em>{ resolved, exact: <strong>true</strong> }</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Otherwise if <em>patternBase</em> is <strong>null</strong> and <em>matchKey</em> starts with
<em>expansionKey</em>, then
<ol>
<li>Let <em>target</em> be the value of <em>matchObj</em>[<em>expansionKey</em>].</li>
<li>Let <em>subpath</em> be the substring of <em>matchKey</em> starting at the
index of the length of <em>expansionKey</em>.</li>
<li>Let <em>resolved</em> be the result of <strong>PACKAGE_TARGET_RESOLVE</strong>(
<em>packageURL</em>, <em>target</em>, <em>subpath</em>, <strong>false</strong>, <em>isImports</em>,
<em>conditions</em>).</li>
<li>Return the object <em>{ resolved, exact: <strong>false</strong> }</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Return the object <em>{ resolved: <strong>null</strong>, exact: <strong>true</strong> }</em>.</li>
</ol>
</blockquote>
<p><strong>PATTERN_KEY_COMPARE</strong>(<em>keyA</em>, <em>keyB</em>)</p>
<blockquote>
<ol>
<li>Assert: <em>keyA</em> ends with <em>"/"</em> or contains only a single <em>"*"</em>.</li>
<li>Assert: <em>keyB</em> ends with <em>"/"</em> or contains only a single <em>"*"</em>.</li>
<li>Let <em>baseLengthA</em> be the index of <em>"*"</em> in <em>keyA</em> plus one, if <em>keyA</em>
contains <em>"*"</em>, or the length of <em>keyA</em> otherwise.</li>
<li>Let <em>baseLengthB</em> be the index of <em>"*"</em> in <em>keyB</em> plus one, if <em>keyB</em>
contains <em>"*"</em>, or the length of <em>keyB</em> otherwise.</li>
<li>If <em>baseLengthA</em> is greater than <em>baseLengthB</em>, return -1.</li>
<li>If <em>baseLengthB</em> is greater than <em>baseLengthA</em>, return 1.</li>
<li>If <em>keyA</em> does not contain <em>"*"</em>, return 1.</li>
<li>If <em>keyB</em> does not contain <em>"*"</em>, return -1.</li>
<li>If the length of <em>keyA</em> is greater than the length of <em>keyB</em>, return -1.</li>
<li>If the length of <em>keyB</em> is greater than the length of <em>keyA</em>, return 1.</li>
<li>Return 0.</li>
</ol>
</blockquote>
<p><strong>PACKAGE_TARGET_RESOLVE</strong>(<em>packageURL</em>, <em>target</em>, <em>subpath</em>, <em>pattern</em>,
<em>internal</em>, <em>conditions</em>)</p>
<blockquote>
<ol>
<li>If <em>target</em> is a String, then
<ol>
<li>If <em>pattern</em> is <strong>false</strong>, <em>subpath</em> has non-zero length and <em>target</em>
does not end with <em>"/"</em>, throw an <em>Invalid Module Specifier</em> error.</li>
<li>If <em>target</em> does not start with <em>"./"</em>, then
<ol>
<li>If <em>internal</em> is <strong>true</strong> and <em>target</em> does not start with <em>"../"</em> or
<em>"/"</em> and is not a valid URL, then
<ol>
<li>If <em>pattern</em> is <strong>true</strong>, then
<ol>
<li>Return <strong>PACKAGE_RESOLVE</strong>(<em>target</em> with every instance of
<em>"*"</em> replaced by <em>subpath</em>, <em>packageURL</em> + <em>"/"</em>).</li>
</ol>
</li>
<li>Return <strong>PACKAGE_RESOLVE</strong>(<em>target</em> + <em>subpath</em>,
<em>packageURL</em> + <em>"/"</em>).</li>
</ol>
</li>
<li>Otherwise, throw an <em>Invalid Package Target</em> error.</li>
</ol>
</li>
<li>If <em>target</em> split on <em>"/"</em> or <em>"\"</em> contains any <em>"."</em>, <em>".."</em> or
<em>"node_modules"</em> segments after the first segment, case insensitive and
including percent encoded variants, throw an <em>Invalid Package Target</em>
error.</li>
<li>Let <em>resolvedTarget</em> be the URL resolution of the concatenation of
<em>packageURL</em> and <em>target</em>.</li>
<li>Assert: <em>resolvedTarget</em> is contained in <em>packageURL</em>.</li>
<li>If <em>subpath</em> split on <em>"/"</em> or <em>"\"</em> contains any <em>"."</em>, <em>".."</em> or
<em>"node_modules"</em> segments, case insensitive and including percent
encoded variants, throw an <em>Invalid Module Specifier</em> error.</li>
<li>If <em>pattern</em> is <strong>true</strong>, then
<ol>
<li>Return the URL resolution of <em>resolvedTarget</em> with every instance of
<em>"*"</em> replaced with <em>subpath</em>.</li>
</ol>
</li>
<li>Otherwise,
<ol>
<li>Return the URL resolution of the concatenation of <em>subpath</em> and
<em>resolvedTarget</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Otherwise, if <em>target</em> is a non-null Object, then
<ol>
<li>If <em>exports</em> contains any index property keys, as defined in ECMA-262
<a href="https://tc39.es/ecma262/#integer-index">6.1.7 Array Index</a>, throw an <em>Invalid Package Configuration</em> error.</li>
<li>For each property <em>p</em> of <em>target</em>, in object insertion order as,
<ol>
<li>If <em>p</em> equals <em>"default"</em> or <em>conditions</em> contains an entry for <em>p</em>,
then
<ol>
<li>Let <em>targetValue</em> be the value of the <em>p</em> property in <em>target</em>.</li>
<li>Let <em>resolved</em> be the result of <strong>PACKAGE_TARGET_RESOLVE</strong>(
<em>packageURL</em>, <em>targetValue</em>, <em>subpath</em>, <em>pattern</em>, <em>internal</em>,
<em>conditions</em>).</li>
<li>If <em>resolved</em> is equal to <strong>undefined</strong>, continue the loop.</li>
<li>Return <em>resolved</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Return <strong>undefined</strong>.</li>
</ol>
</li>
<li>Otherwise, if <em>target</em> is an Array, then
<ol>
<li>If _target.length is zero, return <strong>null</strong>.</li>
<li>For each item <em>targetValue</em> in <em>target</em>, do
<ol>
<li>Let <em>resolved</em> be the result of <strong>PACKAGE_TARGET_RESOLVE</strong>(
<em>packageURL</em>, <em>targetValue</em>, <em>subpath</em>, <em>pattern</em>, <em>internal</em>,
<em>conditions</em>), continuing the loop on any <em>Invalid Package Target</em>
error.</li>
<li>If <em>resolved</em> is <strong>undefined</strong>, continue the loop.</li>
<li>Return <em>resolved</em>.</li>
</ol>
</li>
<li>Return or throw the last fallback resolution <strong>null</strong> return or error.</li>
</ol>
</li>
<li>Otherwise, if <em>target</em> is <em>null</em>, return <strong>null</strong>.</li>
<li>Otherwise throw an <em>Invalid Package Target</em> error.</li>
</ol>
</blockquote>
<p><strong>ESM_FILE_FORMAT</strong>(<em>url</em>)</p>
<blockquote>
<ol>
<li>Assert: <em>url</em> corresponds to an existing file.</li>
<li>If <em>url</em> ends in <em>".mjs"</em>, then
<ol>
<li>Return <em>"module"</em>.</li>
</ol>
</li>
<li>If <em>url</em> ends in <em>".cjs"</em>, then
<ol>
<li>Return <em>"commonjs"</em>.</li>
</ol>
</li>
<li>If <em>url</em> ends in <em>".json"</em>, then
<ol>
<li>Return <em>"json"</em>.</li>
</ol>
</li>
<li>Let <em>packageURL</em> be the result of <strong>LOOKUP_PACKAGE_SCOPE</strong>(<em>url</em>).</li>
<li>Let <em>pjson</em> be the result of <strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>).</li>
<li>If <em>pjson?.type</em> exists and is <em>"module"</em>, then
<ol>
<li>If <em>url</em> ends in <em>".js"</em>, then
<ol>
<li>Return <em>"module"</em>.</li>
</ol>
</li>
<li>Throw an <em>Unsupported File Extension</em> error.</li>
</ol>
</li>
<li>Otherwise,
<ol>
<li>Throw an <em>Unsupported File Extension</em> error.</li>
</ol>
</li>
</ol>
</blockquote>
<p><strong>LOOKUP_PACKAGE_SCOPE</strong>(<em>url</em>)</p>
<blockquote>
<ol>
<li>Let <em>scopeURL</em> be <em>url</em>.</li>
<li>While <em>scopeURL</em> is not the file system root,
<ol>
<li>Set <em>scopeURL</em> to the parent URL of <em>scopeURL</em>.</li>
<li>If <em>scopeURL</em> ends in a <em>"node_modules"</em> path segment, return <strong>null</strong>.</li>
<li>Let <em>pjsonURL</em> be the resolution of <em>"package.json"</em> within
<em>scopeURL</em>.</li>
<li>if the file at <em>pjsonURL</em> exists, then
<ol>
<li>Return <em>scopeURL</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Return <strong>null</strong>.</li>
</ol>
</blockquote>
<p><strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>)</p>
<blockquote>
<ol>
<li>Let <em>pjsonURL</em> be the resolution of <em>"package.json"</em> within <em>packageURL</em>.</li>
<li>If the file at <em>pjsonURL</em> does not exist, then
<ol>
<li>Return <strong>null</strong>.</li>
</ol>
</li>
<li>If the file at <em>packageURL</em> does not parse as valid JSON, then
<ol>
<li>Throw an <em>Invalid Package Configuration</em> error.</li>
</ol>
</li>
<li>Return the parsed JSON source of the file at <em>pjsonURL</em>.</li>
</ol>
</blockquote>
<h4>Customizing ESM specifier resolution algorithm<span><a class="mark" href="#customizing-esm-specifier-resolution-algorithm" id="customizing-esm-specifier-resolution-algorithm">#</a></span><a aria-hidden="true" class="legacy" id="esm_customizing_esm_specifier_resolution_algorithm"></a></h4>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<blockquote>
<p>Do not rely on this flag. We plan to remove it once the
<a href="#loaders">Loaders API</a> has advanced to the point that equivalent functionality can
be achieved via custom loaders.</p>
</blockquote>
<p>The current specifier resolution does not support all default behavior of
the CommonJS loader. One of the behavior differences is automatic resolution
of file extensions and the ability to import directories that have an index
file.</p>
<p>The <code>--experimental-specifier-resolution=[mode]</code> flag can be used to customize
the extension resolution algorithm. The default mode is <code>explicit</code>, which
requires the full path to a module be provided to the loader. To enable the
automatic extension resolution and importing from directories that include an
index file use the <code>node</code> mode.</p>
<pre><code class="language-console"><span class="hljs-meta prompt_">$ </span><span class="language-bash">node index.mjs</span>
success!
<span class="hljs-meta prompt_">$ </span><span class="language-bash">node index <span class="hljs-comment"># Failure!</span></span>
Error: Cannot find module
<span class="hljs-meta prompt_">$ </span><span class="language-bash">node --experimental-specifier-resolution=node index</span>
success!</code></pre>
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