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<details id="toc" open><summary>Table of contents</summary><ul>
<li><a href="#c-addons">C++ addons</a>
<ul>
<li><a href="#hello-world">Hello world</a>
<ul>
<li><a href="#context-aware-addons">Context-aware addons</a>
<ul>
<li><a href="#worker-support">Worker support</a></li>
</ul>
</li>
<li><a href="#building">Building</a></li>
<li><a href="#linking-to-libraries-included-with-nodejs">Linking to libraries included with Node.js</a></li>
<li><a href="#loading-addons-using-require">Loading addons using <code>require()</code></a></li>
</ul>
</li>
<li><a href="#native-abstractions-for-nodejs">Native abstractions for Node.js</a></li>
<li><span class="stability_2"><a href="#node-api">Node-API</a></span></li>
<li><a href="#addon-examples">Addon examples</a>
<ul>
<li><a href="#function-arguments">Function arguments</a></li>
<li><a href="#callbacks">Callbacks</a></li>
<li><a href="#object-factory">Object factory</a></li>
<li><a href="#function-factory">Function factory</a></li>
<li><a href="#wrapping-c-objects">Wrapping C++ objects</a></li>
<li><a href="#factory-of-wrapped-objects">Factory of wrapped objects</a></li>
<li><a href="#passing-wrapped-objects-around">Passing wrapped objects around</a></li>
</ul>
</li>
</ul>
</li>
</ul></details>
<div id="apicontent">
<h2>C++ addons<span><a class="mark" href="#c-addons" id="c-addons">#</a></span><a aria-hidden="true" class="legacy" id="addons_c_addons"></a></h2>
<p><em>Addons</em> are dynamically-linked shared objects written in C++. The
<a href="modules.html#requireid"><code>require()</code></a> function can load addons as ordinary Node.js modules.
Addons provide an interface between JavaScript and C/C++ libraries.</p>
<p>There are three options for implementing addons: Node-API, nan, or direct
use of internal V8, libuv and Node.js libraries. Unless there is a need for
direct access to functionality which is not exposed by Node-API, use Node-API.
Refer to <a href="n-api.html">C/C++ addons with Node-API</a> for more information on
Node-API.</p>
<p>When not using Node-API, implementing addons is complicated,
involving knowledge of several components and APIs:</p>
<ul>
<li>
<p><a href="https://v8.dev/">V8</a>: the C++ library Node.js uses to provide the
JavaScript implementation. V8 provides the mechanisms for creating objects,
calling functions, etc. V8's API is documented mostly in the
<code>v8.h</code> header file (<code>deps/v8/include/v8.h</code> in the Node.js source
tree), which is also available <a href="https://v8docs.nodesource.com/">online</a>.</p>
</li>
<li>
<p><a href="https://github.com/libuv/libuv">libuv</a>: The C library that implements the Node.js event loop, its worker
threads and all of the asynchronous behaviors of the platform. It also
serves as a cross-platform abstraction library, giving easy, POSIX-like
access across all major operating systems to many common system tasks, such
as interacting with the filesystem, sockets, timers, and system events. libuv
also provides a threading abstraction similar to POSIX threads for
more sophisticated asynchronous addons that need to move beyond the
standard event loop. Addon authors should
avoid blocking the event loop with I/O or other time-intensive tasks by
offloading work via libuv to non-blocking system operations, worker threads,
or a custom use of libuv threads.</p>
</li>
<li>
<p>Internal Node.js libraries. Node.js itself exports C++ APIs that addons can
use, the most important of which is the <code>node::ObjectWrap</code> class.</p>
</li>
<li>
<p>Node.js includes other statically linked libraries including OpenSSL. These
other libraries are located in the <code>deps/</code> directory in the Node.js source
tree. Only the libuv, OpenSSL, V8 and zlib symbols are purposefully
re-exported by Node.js and may be used to various extents by addons. See
<a href="#linking-to-libraries-included-with-nodejs">Linking to libraries included with Node.js</a> for additional information.</p>
</li>
</ul>
<p>All of the following examples are available for <a href="https://github.com/nodejs/node-addon-examples">download</a> and may
be used as the starting-point for an addon.</p>
<section><h3>Hello world<span><a class="mark" href="#hello-world" id="hello-world">#</a></span><a aria-hidden="true" class="legacy" id="addons_hello_world"></a></h3>
<p>This "Hello world" example is a simple addon, written in C++, that is the
equivalent of the following JavaScript code:</p>
<pre><code class="language-js"><span class="hljs-variable language_">module</span>.<span class="hljs-property">exports</span>.<span class="hljs-property">hello</span> = <span class="hljs-function">() =></span> <span class="hljs-string">'world'</span>;</code></pre>
<p>First, create the file <code>hello.cc</code>:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// hello.cc</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">using</span> v8::FunctionCallbackInfo;
<span class="hljs-keyword">using</span> v8::Isolate;
<span class="hljs-keyword">using</span> v8::Local;
<span class="hljs-keyword">using</span> v8::Object;
<span class="hljs-keyword">using</span> v8::String;
<span class="hljs-keyword">using</span> v8::Value;
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">Method</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(String::<span class="hljs-built_in">NewFromUtf8</span>(
isolate, <span class="hljs-string">"world"</span>).<span class="hljs-built_in">ToLocalChecked</span>());
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">Initialize</span><span class="hljs-params">(Local<Object> exports)</span> </span>{
<span class="hljs-built_in">NODE_SET_METHOD</span>(exports, <span class="hljs-string">"hello"</span>, Method);
}
<span class="hljs-built_in">NODE_MODULE</span>(NODE_GYP_MODULE_NAME, Initialize)
} <span class="hljs-comment">// namespace demo</span></code></pre>
<p>All Node.js addons must export an initialization function following
the pattern:</p>
<pre><code class="language-cpp"><span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">Initialize</span><span class="hljs-params">(Local<Object> exports)</span></span>;
<span class="hljs-built_in">NODE_MODULE</span>(NODE_GYP_MODULE_NAME, Initialize)</code></pre>
<p>There is no semi-colon after <code>NODE_MODULE</code> as it's not a function (see
<code>node.h</code>).</p>
<p>The <code>module_name</code> must match the filename of the final binary (excluding
the <code>.node</code> suffix).</p>
<p>In the <code>hello.cc</code> example, then, the initialization function is <code>Initialize</code>
and the addon module name is <code>addon</code>.</p>
<p>When building addons with <code>node-gyp</code>, using the macro <code>NODE_GYP_MODULE_NAME</code> as
the first parameter of <code>NODE_MODULE()</code> will ensure that the name of the final
binary will be passed to <code>NODE_MODULE()</code>.</p>
<h4>Context-aware addons<span><a class="mark" href="#context-aware-addons" id="context-aware-addons">#</a></span><a aria-hidden="true" class="legacy" id="addons_context_aware_addons"></a></h4>
<p>There are environments in which Node.js addons may need to be loaded multiple
times in multiple contexts. For example, the <a href="https://electronjs.org/">Electron</a> runtime runs multiple
instances of Node.js in a single process. Each instance will have its own
<code>require()</code> cache, and thus each instance will need a native addon to behave
correctly when loaded via <code>require()</code>. This means that the addon
must support multiple initializations.</p>
<p>A context-aware addon can be constructed by using the macro
<code>NODE_MODULE_INITIALIZER</code>, which expands to the name of a function which Node.js
will expect to find when it loads an addon. An addon can thus be initialized as
in the following example:</p>
<pre><code class="language-cpp"><span class="hljs-keyword">using</span> <span class="hljs-keyword">namespace</span> v8;
<span class="hljs-keyword">extern</span> <span class="hljs-string">"C"</span> <span class="hljs-function">NODE_MODULE_EXPORT <span class="hljs-type">void</span>
<span class="hljs-title">NODE_MODULE_INITIALIZER</span><span class="hljs-params">(Local<Object> exports,
Local<Value> <span class="hljs-keyword">module</span>,
Local<Context> context)</span> </span>{
<span class="hljs-comment">/* Perform addon initialization steps here. */</span>
}</code></pre>
<p>Another option is to use the macro <code>NODE_MODULE_INIT()</code>, which will also
construct a context-aware addon. Unlike <code>NODE_MODULE()</code>, which is used to
construct an addon around a given addon initializer function,
<code>NODE_MODULE_INIT()</code> serves as the declaration of such an initializer to be
followed by a function body.</p>
<p>The following three variables may be used inside the function body following an
invocation of <code>NODE_MODULE_INIT()</code>:</p>
<ul>
<li><code>Local<Object> exports</code>,</li>
<li><code>Local<Value> module</code>, and</li>
<li><code>Local<Context> context</code></li>
</ul>
<p>The choice to build a context-aware addon carries with it the responsibility of
carefully managing global static data. Since the addon may be loaded multiple
times, potentially even from different threads, any global static data stored
in the addon must be properly protected, and must not contain any persistent
references to JavaScript objects. The reason for this is that JavaScript
objects are only valid in one context, and will likely cause a crash when
accessed from the wrong context or from a different thread than the one on which
they were created.</p>
<p>The context-aware addon can be structured to avoid global static data by
performing the following steps:</p>
<ul>
<li>Define a class which will hold per-addon-instance data and which has a static
member of the form
<pre><code class="language-cpp"><span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">DeleteInstance</span><span class="hljs-params">(<span class="hljs-type">void</span>* data)</span> </span>{
<span class="hljs-comment">// Cast `data` to an instance of the class and delete it.</span>
}</code></pre>
</li>
<li>Heap-allocate an instance of this class in the addon initializer. This can be
accomplished using the <code>new</code> keyword.</li>
<li>Call <code>node::AddEnvironmentCleanupHook()</code>, passing it the above-created
instance and a pointer to <code>DeleteInstance()</code>. This will ensure the instance is
deleted when the environment is torn down.</li>
<li>Store the instance of the class in a <code>v8::External</code>, and</li>
<li>Pass the <code>v8::External</code> to all methods exposed to JavaScript by passing it
to <code>v8::FunctionTemplate::New()</code> or <code>v8::Function::New()</code> which creates the
native-backed JavaScript functions. The third parameter of
<code>v8::FunctionTemplate::New()</code> or <code>v8::Function::New()</code> accepts the
<code>v8::External</code> and makes it available in the native callback using the
<code>v8::FunctionCallbackInfo::Data()</code> method.</li>
</ul>
<p>This will ensure that the per-addon-instance data reaches each binding that can
be called from JavaScript. The per-addon-instance data must also be passed into
any asynchronous callbacks the addon may create.</p>
<p>The following example illustrates the implementation of a context-aware addon:</p>
<pre><code class="language-cpp"><span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-keyword">using</span> <span class="hljs-keyword">namespace</span> v8;
<span class="hljs-keyword">class</span> <span class="hljs-title class_">AddonData</span> {
<span class="hljs-keyword">public</span>:
<span class="hljs-function"><span class="hljs-keyword">explicit</span> <span class="hljs-title">AddonData</span><span class="hljs-params">(Isolate* isolate)</span>:
call_count(<span class="hljs-number">0</span>) {</span>
<span class="hljs-comment">// Ensure this per-addon-instance data is deleted at environment cleanup.</span>
node::<span class="hljs-built_in">AddEnvironmentCleanupHook</span>(isolate, DeleteInstance, <span class="hljs-keyword">this</span>);
}
<span class="hljs-comment">// Per-addon data.</span>
<span class="hljs-type">int</span> call_count;
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">DeleteInstance</span><span class="hljs-params">(<span class="hljs-type">void</span>* data)</span> </span>{
<span class="hljs-keyword">delete</span> <span class="hljs-built_in">static_cast</span><AddonData*>(data);
}
};
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">Method</span><span class="hljs-params">(<span class="hljs-type">const</span> v8::FunctionCallbackInfo<v8::Value>& info)</span> </span>{
<span class="hljs-comment">// Retrieve the per-addon-instance data.</span>
AddonData* data =
<span class="hljs-built_in">reinterpret_cast</span><AddonData*>(info.<span class="hljs-built_in">Data</span>().<span class="hljs-built_in">As</span><External>()-><span class="hljs-built_in">Value</span>());
data->call_count++;
info.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>((<span class="hljs-type">double</span>)data->call_count);
}
<span class="hljs-comment">// Initialize this addon to be context-aware.</span>
<span class="hljs-built_in">NODE_MODULE_INIT</span>(<span class="hljs-comment">/* exports, module, context */</span>) {
Isolate* isolate = context-><span class="hljs-built_in">GetIsolate</span>();
<span class="hljs-comment">// Create a new instance of `AddonData` for this instance of the addon and</span>
<span class="hljs-comment">// tie its life cycle to that of the Node.js environment.</span>
AddonData* data = <span class="hljs-keyword">new</span> <span class="hljs-built_in">AddonData</span>(isolate);
<span class="hljs-comment">// Wrap the data in a `v8::External` so we can pass it to the method we</span>
<span class="hljs-comment">// expose.</span>
Local<External> external = External::<span class="hljs-built_in">New</span>(isolate, data);
<span class="hljs-comment">// Expose the method `Method` to JavaScript, and make sure it receives the</span>
<span class="hljs-comment">// per-addon-instance data we created above by passing `external` as the</span>
<span class="hljs-comment">// third parameter to the `FunctionTemplate` constructor.</span>
exports-><span class="hljs-built_in">Set</span>(context,
String::<span class="hljs-built_in">NewFromUtf8</span>(isolate, <span class="hljs-string">"method"</span>).<span class="hljs-built_in">ToLocalChecked</span>(),
FunctionTemplate::<span class="hljs-built_in">New</span>(isolate, Method, external)
-><span class="hljs-built_in">GetFunction</span>(context).<span class="hljs-built_in">ToLocalChecked</span>()).<span class="hljs-built_in">FromJust</span>();
}</code></pre>
<h5>Worker support<span><a class="mark" href="#worker-support" id="worker-support">#</a></span><a aria-hidden="true" class="legacy" id="addons_worker_support"></a></h5>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v14.8.0, v12.19.0</td>
<td><p>Cleanup hooks may now be asynchronous.</p></td></tr>
</tbody></table>
</details>
</div>
<p>In order to be loaded from multiple Node.js environments,
such as a main thread and a Worker thread, an add-on needs to either:</p>
<ul>
<li>Be an Node-API addon, or</li>
<li>Be declared as context-aware using <code>NODE_MODULE_INIT()</code> as described above</li>
</ul>
<p>In order to support <a href="worker_threads.html#class-worker"><code>Worker</code></a> threads, addons need to clean up any resources
they may have allocated when such a thread exists. This can be achieved through
the usage of the <code>AddEnvironmentCleanupHook()</code> function:</p>
<pre><code class="language-cpp"><span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">AddEnvironmentCleanupHook</span><span class="hljs-params">(v8::Isolate* isolate,
<span class="hljs-type">void</span> (*fun)(<span class="hljs-type">void</span>* arg),
<span class="hljs-type">void</span>* arg)</span></span>;</code></pre>
<p>This function adds a hook that will run before a given Node.js instance shuts
down. If necessary, such hooks can be removed before they are run using
<code>RemoveEnvironmentCleanupHook()</code>, which has the same signature. Callbacks are
run in last-in first-out order.</p>
<p>If necessary, there is an additional pair of <code>AddEnvironmentCleanupHook()</code>
and <code>RemoveEnvironmentCleanupHook()</code> overloads, where the cleanup hook takes a
callback function. This can be used for shutting down asynchronous resources,
such as any libuv handles registered by the addon.</p>
<p>The following <code>addon.cc</code> uses <code>AddEnvironmentCleanupHook</code>:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// addon.cc</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><assert.h></span></span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><stdlib.h></span></span>
<span class="hljs-keyword">using</span> node::AddEnvironmentCleanupHook;
<span class="hljs-keyword">using</span> v8::HandleScope;
<span class="hljs-keyword">using</span> v8::Isolate;
<span class="hljs-keyword">using</span> v8::Local;
<span class="hljs-keyword">using</span> v8::Object;
<span class="hljs-comment">// Note: In a real-world application, do not rely on static/global data.</span>
<span class="hljs-type">static</span> <span class="hljs-type">char</span> cookie[] = <span class="hljs-string">"yum yum"</span>;
<span class="hljs-type">static</span> <span class="hljs-type">int</span> cleanup_cb1_called = <span class="hljs-number">0</span>;
<span class="hljs-type">static</span> <span class="hljs-type">int</span> cleanup_cb2_called = <span class="hljs-number">0</span>;
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">cleanup_cb1</span><span class="hljs-params">(<span class="hljs-type">void</span>* arg)</span> </span>{
Isolate* isolate = <span class="hljs-built_in">static_cast</span><Isolate*>(arg);
<span class="hljs-function">HandleScope <span class="hljs-title">scope</span><span class="hljs-params">(isolate)</span></span>;
Local<Object> obj = Object::<span class="hljs-built_in">New</span>(isolate);
<span class="hljs-built_in">assert</span>(!obj.<span class="hljs-built_in">IsEmpty</span>()); <span class="hljs-comment">// assert VM is still alive</span>
<span class="hljs-built_in">assert</span>(obj-><span class="hljs-built_in">IsObject</span>());
cleanup_cb1_called++;
}
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">cleanup_cb2</span><span class="hljs-params">(<span class="hljs-type">void</span>* arg)</span> </span>{
<span class="hljs-built_in">assert</span>(arg == <span class="hljs-built_in">static_cast</span><<span class="hljs-type">void</span>*>(cookie));
cleanup_cb2_called++;
}
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">sanity_check</span><span class="hljs-params">(<span class="hljs-type">void</span>*)</span> </span>{
<span class="hljs-built_in">assert</span>(cleanup_cb1_called == <span class="hljs-number">1</span>);
<span class="hljs-built_in">assert</span>(cleanup_cb2_called == <span class="hljs-number">1</span>);
}
<span class="hljs-comment">// Initialize this addon to be context-aware.</span>
<span class="hljs-built_in">NODE_MODULE_INIT</span>(<span class="hljs-comment">/* exports, module, context */</span>) {
Isolate* isolate = context-><span class="hljs-built_in">GetIsolate</span>();
<span class="hljs-built_in">AddEnvironmentCleanupHook</span>(isolate, sanity_check, <span class="hljs-literal">nullptr</span>);
<span class="hljs-built_in">AddEnvironmentCleanupHook</span>(isolate, cleanup_cb2, cookie);
<span class="hljs-built_in">AddEnvironmentCleanupHook</span>(isolate, cleanup_cb1, isolate);
}</code></pre>
<p>Test in JavaScript by running:</p>
<pre><code class="language-js"><span class="hljs-comment">// test.js</span>
<span class="hljs-built_in">require</span>(<span class="hljs-string">'./build/Release/addon'</span>);</code></pre>
<h4>Building<span><a class="mark" href="#building" id="building">#</a></span><a aria-hidden="true" class="legacy" id="addons_building"></a></h4>
<p>Once the source code has been written, it must be compiled into the binary
<code>addon.node</code> file. To do so, create a file called <code>binding.gyp</code> in the
top-level of the project describing the build configuration of the module
using a JSON-like format. This file is used by <a href="https://github.com/nodejs/node-gyp">node-gyp</a>, a tool written
specifically to compile Node.js addons.</p>
<pre><code class="language-json"><span class="hljs-punctuation">{</span>
<span class="hljs-attr">"targets"</span><span class="hljs-punctuation">:</span> <span class="hljs-punctuation">[</span>
<span class="hljs-punctuation">{</span>
<span class="hljs-attr">"target_name"</span><span class="hljs-punctuation">:</span> <span class="hljs-string">"addon"</span><span class="hljs-punctuation">,</span>
<span class="hljs-attr">"sources"</span><span class="hljs-punctuation">:</span> <span class="hljs-punctuation">[</span> <span class="hljs-string">"hello.cc"</span> <span class="hljs-punctuation">]</span>
<span class="hljs-punctuation">}</span>
<span class="hljs-punctuation">]</span>
<span class="hljs-punctuation">}</span></code></pre>
<p>A version of the <code>node-gyp</code> utility is bundled and distributed with
Node.js as part of <code>npm</code>. This version is not made directly available for
developers to use and is intended only to support the ability to use the
<code>npm install</code> command to compile and install addons. Developers who wish to
use <code>node-gyp</code> directly can install it using the command
<code>npm install -g node-gyp</code>. See the <code>node-gyp</code> <a href="https://github.com/nodejs/node-gyp#installation">installation instructions</a> for
more information, including platform-specific requirements.</p>
<p>Once the <code>binding.gyp</code> file has been created, use <code>node-gyp configure</code> to
generate the appropriate project build files for the current platform. This
will generate either a <code>Makefile</code> (on Unix platforms) or a <code>vcxproj</code> file
(on Windows) in the <code>build/</code> directory.</p>
<p>Next, invoke the <code>node-gyp build</code> command to generate the compiled <code>addon.node</code>
file. This will be put into the <code>build/Release/</code> directory.</p>
<p>When using <code>npm install</code> to install a Node.js addon, npm uses its own bundled
version of <code>node-gyp</code> to perform this same set of actions, generating a
compiled version of the addon for the user's platform on demand.</p>
<p>Once built, the binary addon can be used from within Node.js by pointing
<a href="modules.html#requireid"><code>require()</code></a> to the built <code>addon.node</code> module:</p>
<pre><code class="language-js"><span class="hljs-comment">// hello.js</span>
<span class="hljs-keyword">const</span> addon = <span class="hljs-built_in">require</span>(<span class="hljs-string">'./build/Release/addon'</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(addon.<span class="hljs-title function_">hello</span>());
<span class="hljs-comment">// Prints: 'world'</span></code></pre>
<p>Because the exact path to the compiled addon binary can vary depending on how
it is compiled (i.e. sometimes it may be in <code>./build/Debug/</code>), addons can use
the <a href="https://github.com/TooTallNate/node-bindings">bindings</a> package to load the compiled module.</p>
<p>While the <code>bindings</code> package implementation is more sophisticated in how it
locates addon modules, it is essentially using a <code>try…catch</code> pattern similar to:</p>
<pre><code class="language-js"><span class="hljs-keyword">try</span> {
<span class="hljs-keyword">return</span> <span class="hljs-built_in">require</span>(<span class="hljs-string">'./build/Release/addon.node'</span>);
} <span class="hljs-keyword">catch</span> (err) {
<span class="hljs-keyword">return</span> <span class="hljs-built_in">require</span>(<span class="hljs-string">'./build/Debug/addon.node'</span>);
}</code></pre>
<h4>Linking to libraries included with Node.js<span><a class="mark" href="#linking-to-libraries-included-with-nodejs" id="linking-to-libraries-included-with-nodejs">#</a></span><a aria-hidden="true" class="legacy" id="addons_linking_to_libraries_included_with_node_js"></a></h4>
<p>Node.js uses statically linked libraries such as V8, libuv and OpenSSL. All
addons are required to link to V8 and may link to any of the other dependencies
as well. Typically, this is as simple as including the appropriate
<code>#include <...></code> statements (e.g. <code>#include <v8.h></code>) and <code>node-gyp</code> will locate
the appropriate headers automatically. However, there are a few caveats to be
aware of:</p>
<ul>
<li>
<p>When <code>node-gyp</code> runs, it will detect the specific release version of Node.js
and download either the full source tarball or just the headers. If the full
source is downloaded, addons will have complete access to the full set of
Node.js dependencies. However, if only the Node.js headers are downloaded,
then only the symbols exported by Node.js will be available.</p>
</li>
<li>
<p><code>node-gyp</code> can be run using the <code>--nodedir</code> flag pointing at a local Node.js
source image. Using this option, the addon will have access to the full set of
dependencies.</p>
</li>
</ul>
<h4>Loading addons using <code>require()</code><span><a class="mark" href="#loading-addons-using-require" id="loading-addons-using-require">#</a></span><a aria-hidden="true" class="legacy" id="addons_loading_addons_using_require"></a></h4>
<p>The filename extension of the compiled addon binary is <code>.node</code> (as opposed
to <code>.dll</code> or <code>.so</code>). The <a href="modules.html#requireid"><code>require()</code></a> function is written to look for
files with the <code>.node</code> file extension and initialize those as dynamically-linked
libraries.</p>
<p>When calling <a href="modules.html#requireid"><code>require()</code></a>, the <code>.node</code> extension can usually be
omitted and Node.js will still find and initialize the addon. One caveat,
however, is that Node.js will first attempt to locate and load modules or
JavaScript files that happen to share the same base name. For instance, if
there is a file <code>addon.js</code> in the same directory as the binary <code>addon.node</code>,
then <a href="modules.html#requireid"><code>require('addon')</code></a> will give precedence to the <code>addon.js</code> file
and load it instead.</p>
</section><section><h3>Native abstractions for Node.js<span><a class="mark" href="#native-abstractions-for-nodejs" id="native-abstractions-for-nodejs">#</a></span><a aria-hidden="true" class="legacy" id="addons_native_abstractions_for_node_js"></a></h3>
<p>Each of the examples illustrated in this document directly use the
Node.js and V8 APIs for implementing addons. The V8 API can, and has, changed
dramatically from one V8 release to the next (and one major Node.js release to
the next). With each change, addons may need to be updated and recompiled in
order to continue functioning. The Node.js release schedule is designed to
minimize the frequency and impact of such changes but there is little that
Node.js can do to ensure stability of the V8 APIs.</p>
<p>The <a href="https://github.com/nodejs/nan">Native Abstractions for Node.js</a> (or <code>nan</code>) provide a set of tools that
addon developers are recommended to use to keep compatibility between past and
future releases of V8 and Node.js. See the <code>nan</code> <a href="https://github.com/nodejs/nan/tree/HEAD/examples/">examples</a> for an
illustration of how it can be used.</p>
</section><section><h3>Node-API<span><a class="mark" href="#node-api" id="node-api">#</a></span><a aria-hidden="true" class="legacy" id="addons_node_api"></a></h3>
<p></p><div class="api_stability api_stability_2"><a href="documentation.html#stability-index">Stability: 2</a> - Stable</div><p></p>
<p>Node-API is an API for building native addons. It is independent from
the underlying JavaScript runtime (e.g. V8) and is maintained as part of
Node.js itself. This API will be Application Binary Interface (ABI) stable
across versions of Node.js. It is intended to insulate addons from
changes in the underlying JavaScript engine and allow modules
compiled for one version to run on later versions of Node.js without
recompilation. Addons are built/packaged with the same approach/tools
outlined in this document (node-gyp, etc.). The only difference is the
set of APIs that are used by the native code. Instead of using the V8
or <a href="https://github.com/nodejs/nan">Native Abstractions for Node.js</a> APIs, the functions available
in the Node-API are used.</p>
<p>Creating and maintaining an addon that benefits from the ABI stability
provided by Node-API carries with it certain
<a href="n-api.html#implications-of-abi-stability">implementation considerations</a>.</p>
<p>To use Node-API in the above "Hello world" example, replace the content of
<code>hello.cc</code> with the following. All other instructions remain the same.</p>
<pre><code class="language-cpp"><span class="hljs-comment">// hello.cc using Node-API</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node_api.h></span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-function">napi_value <span class="hljs-title">Method</span><span class="hljs-params">(napi_env env, napi_callback_info args)</span> </span>{
napi_value greeting;
napi_status status;
status = <span class="hljs-built_in">napi_create_string_utf8</span>(env, <span class="hljs-string">"world"</span>, NAPI_AUTO_LENGTH, &greeting);
<span class="hljs-keyword">if</span> (status != napi_ok) <span class="hljs-keyword">return</span> <span class="hljs-literal">nullptr</span>;
<span class="hljs-keyword">return</span> greeting;
}
<span class="hljs-function">napi_value <span class="hljs-title">init</span><span class="hljs-params">(napi_env env, napi_value exports)</span> </span>{
napi_status status;
napi_value fn;
status = <span class="hljs-built_in">napi_create_function</span>(env, <span class="hljs-literal">nullptr</span>, <span class="hljs-number">0</span>, Method, <span class="hljs-literal">nullptr</span>, &fn);
<span class="hljs-keyword">if</span> (status != napi_ok) <span class="hljs-keyword">return</span> <span class="hljs-literal">nullptr</span>;
status = <span class="hljs-built_in">napi_set_named_property</span>(env, exports, <span class="hljs-string">"hello"</span>, fn);
<span class="hljs-keyword">if</span> (status != napi_ok) <span class="hljs-keyword">return</span> <span class="hljs-literal">nullptr</span>;
<span class="hljs-keyword">return</span> exports;
}
<span class="hljs-built_in">NAPI_MODULE</span>(NODE_GYP_MODULE_NAME, init)
} <span class="hljs-comment">// namespace demo</span></code></pre>
<p>The functions available and how to use them are documented in
<a href="n-api.html">C/C++ addons with Node-API</a>.</p>
</section><section><h3>Addon examples<span><a class="mark" href="#addon-examples" id="addon-examples">#</a></span><a aria-hidden="true" class="legacy" id="addons_addon_examples"></a></h3>
<p>Following are some example addons intended to help developers get started. The
examples use the V8 APIs. Refer to the online <a href="https://v8docs.nodesource.com/">V8 reference</a>
for help with the various V8 calls, and V8's <a href="https://v8.dev/docs/embed">Embedder's Guide</a> for an
explanation of several concepts used such as handles, scopes, function
templates, etc.</p>
<p>Each of these examples using the following <code>binding.gyp</code> file:</p>
<pre><code class="language-json"><span class="hljs-punctuation">{</span>
<span class="hljs-attr">"targets"</span><span class="hljs-punctuation">:</span> <span class="hljs-punctuation">[</span>
<span class="hljs-punctuation">{</span>
<span class="hljs-attr">"target_name"</span><span class="hljs-punctuation">:</span> <span class="hljs-string">"addon"</span><span class="hljs-punctuation">,</span>
<span class="hljs-attr">"sources"</span><span class="hljs-punctuation">:</span> <span class="hljs-punctuation">[</span> <span class="hljs-string">"addon.cc"</span> <span class="hljs-punctuation">]</span>
<span class="hljs-punctuation">}</span>
<span class="hljs-punctuation">]</span>
<span class="hljs-punctuation">}</span></code></pre>
<p>In cases where there is more than one <code>.cc</code> file, simply add the additional
filename to the <code>sources</code> array:</p>
<pre><code class="language-json"><span class="hljs-attr">"sources"</span><span class="hljs-punctuation">:</span> <span class="hljs-punctuation">[</span><span class="hljs-string">"addon.cc"</span><span class="hljs-punctuation">,</span> <span class="hljs-string">"myexample.cc"</span><span class="hljs-punctuation">]</span></code></pre>
<p>Once the <code>binding.gyp</code> file is ready, the example addons can be configured and
built using <code>node-gyp</code>:</p>
<pre><code class="language-console"><span class="hljs-meta prompt_">$ </span><span class="language-bash">node-gyp configure build</span></code></pre>
<h4>Function arguments<span><a class="mark" href="#function-arguments" id="function-arguments">#</a></span><a aria-hidden="true" class="legacy" id="addons_function_arguments"></a></h4>
<p>Addons will typically expose objects and functions that can be accessed from
JavaScript running within Node.js. When functions are invoked from JavaScript,
the input arguments and return value must be mapped to and from the C/C++
code.</p>
<p>The following example illustrates how to read function arguments passed from
JavaScript and how to return a result:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// addon.cc</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">using</span> v8::Exception;
<span class="hljs-keyword">using</span> v8::FunctionCallbackInfo;
<span class="hljs-keyword">using</span> v8::Isolate;
<span class="hljs-keyword">using</span> v8::Local;
<span class="hljs-keyword">using</span> v8::Number;
<span class="hljs-keyword">using</span> v8::Object;
<span class="hljs-keyword">using</span> v8::String;
<span class="hljs-keyword">using</span> v8::Value;
<span class="hljs-comment">// This is the implementation of the "add" method</span>
<span class="hljs-comment">// Input arguments are passed using the</span>
<span class="hljs-comment">// const FunctionCallbackInfo<Value>& args struct</span>
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">Add</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
<span class="hljs-comment">// Check the number of arguments passed.</span>
<span class="hljs-keyword">if</span> (args.<span class="hljs-built_in">Length</span>() < <span class="hljs-number">2</span>) {
<span class="hljs-comment">// Throw an Error that is passed back to JavaScript</span>
isolate-><span class="hljs-built_in">ThrowException</span>(Exception::<span class="hljs-built_in">TypeError</span>(
String::<span class="hljs-built_in">NewFromUtf8</span>(isolate,
<span class="hljs-string">"Wrong number of arguments"</span>).<span class="hljs-built_in">ToLocalChecked</span>()));
<span class="hljs-keyword">return</span>;
}
<span class="hljs-comment">// Check the argument types</span>
<span class="hljs-keyword">if</span> (!args[<span class="hljs-number">0</span>]-><span class="hljs-built_in">IsNumber</span>() || !args[<span class="hljs-number">1</span>]-><span class="hljs-built_in">IsNumber</span>()) {
isolate-><span class="hljs-built_in">ThrowException</span>(Exception::<span class="hljs-built_in">TypeError</span>(
String::<span class="hljs-built_in">NewFromUtf8</span>(isolate,
<span class="hljs-string">"Wrong arguments"</span>).<span class="hljs-built_in">ToLocalChecked</span>()));
<span class="hljs-keyword">return</span>;
}
<span class="hljs-comment">// Perform the operation</span>
<span class="hljs-type">double</span> value =
args[<span class="hljs-number">0</span>].<span class="hljs-built_in">As</span><Number>()-><span class="hljs-built_in">Value</span>() + args[<span class="hljs-number">1</span>].<span class="hljs-built_in">As</span><Number>()-><span class="hljs-built_in">Value</span>();
Local<Number> num = Number::<span class="hljs-built_in">New</span>(isolate, value);
<span class="hljs-comment">// Set the return value (using the passed in</span>
<span class="hljs-comment">// FunctionCallbackInfo<Value>&)</span>
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(num);
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">Init</span><span class="hljs-params">(Local<Object> exports)</span> </span>{
<span class="hljs-built_in">NODE_SET_METHOD</span>(exports, <span class="hljs-string">"add"</span>, Add);
}
<span class="hljs-built_in">NODE_MODULE</span>(NODE_GYP_MODULE_NAME, Init)
} <span class="hljs-comment">// namespace demo</span></code></pre>
<p>Once compiled, the example addon can be required and used from within Node.js:</p>
<pre><code class="language-js"><span class="hljs-comment">// test.js</span>
<span class="hljs-keyword">const</span> addon = <span class="hljs-built_in">require</span>(<span class="hljs-string">'./build/Release/addon'</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'This should be eight:'</span>, addon.<span class="hljs-title function_">add</span>(<span class="hljs-number">3</span>, <span class="hljs-number">5</span>));</code></pre>
<h4>Callbacks<span><a class="mark" href="#callbacks" id="callbacks">#</a></span><a aria-hidden="true" class="legacy" id="addons_callbacks"></a></h4>
<p>It is common practice within addons to pass JavaScript functions to a C++
function and execute them from there. The following example illustrates how
to invoke such callbacks:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// addon.cc</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">using</span> v8::Context;
<span class="hljs-keyword">using</span> v8::Function;
<span class="hljs-keyword">using</span> v8::FunctionCallbackInfo;
<span class="hljs-keyword">using</span> v8::Isolate;
<span class="hljs-keyword">using</span> v8::Local;
<span class="hljs-keyword">using</span> v8::Null;
<span class="hljs-keyword">using</span> v8::Object;
<span class="hljs-keyword">using</span> v8::String;
<span class="hljs-keyword">using</span> v8::Value;
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">RunCallback</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
Local<Context> context = isolate-><span class="hljs-built_in">GetCurrentContext</span>();
Local<Function> cb = Local<Function>::<span class="hljs-built_in">Cast</span>(args[<span class="hljs-number">0</span>]);
<span class="hljs-type">const</span> <span class="hljs-type">unsigned</span> argc = <span class="hljs-number">1</span>;
Local<Value> argv[argc] = {
String::<span class="hljs-built_in">NewFromUtf8</span>(isolate,
<span class="hljs-string">"hello world"</span>).<span class="hljs-built_in">ToLocalChecked</span>() };
cb-><span class="hljs-built_in">Call</span>(context, <span class="hljs-built_in">Null</span>(isolate), argc, argv).<span class="hljs-built_in">ToLocalChecked</span>();
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">Init</span><span class="hljs-params">(Local<Object> exports, Local<Object> <span class="hljs-keyword">module</span>)</span> </span>{
<span class="hljs-built_in">NODE_SET_METHOD</span>(<span class="hljs-keyword">module</span>, <span class="hljs-string">"exports"</span>, RunCallback);
}
<span class="hljs-built_in">NODE_MODULE</span>(NODE_GYP_MODULE_NAME, Init)
} <span class="hljs-comment">// namespace demo</span></code></pre>
<p>This example uses a two-argument form of <code>Init()</code> that receives the full
<code>module</code> object as the second argument. This allows the addon to completely
overwrite <code>exports</code> with a single function instead of adding the function as a
property of <code>exports</code>.</p>
<p>To test it, run the following JavaScript:</p>
<pre><code class="language-js"><span class="hljs-comment">// test.js</span>
<span class="hljs-keyword">const</span> addon = <span class="hljs-built_in">require</span>(<span class="hljs-string">'./build/Release/addon'</span>);
<span class="hljs-title function_">addon</span>(<span class="hljs-function">(<span class="hljs-params">msg</span>) =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(msg);
<span class="hljs-comment">// Prints: 'hello world'</span>
});</code></pre>
<p>In this example, the callback function is invoked synchronously.</p>
<h4>Object factory<span><a class="mark" href="#object-factory" id="object-factory">#</a></span><a aria-hidden="true" class="legacy" id="addons_object_factory"></a></h4>
<p>Addons can create and return new objects from within a C++ function as
illustrated in the following example. An object is created and returned with a
property <code>msg</code> that echoes the string passed to <code>createObject()</code>:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// addon.cc</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">using</span> v8::Context;
<span class="hljs-keyword">using</span> v8::FunctionCallbackInfo;
<span class="hljs-keyword">using</span> v8::Isolate;
<span class="hljs-keyword">using</span> v8::Local;
<span class="hljs-keyword">using</span> v8::Object;
<span class="hljs-keyword">using</span> v8::String;
<span class="hljs-keyword">using</span> v8::Value;
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">CreateObject</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
Local<Context> context = isolate-><span class="hljs-built_in">GetCurrentContext</span>();
Local<Object> obj = Object::<span class="hljs-built_in">New</span>(isolate);
obj-><span class="hljs-built_in">Set</span>(context,
String::<span class="hljs-built_in">NewFromUtf8</span>(isolate,
<span class="hljs-string">"msg"</span>).<span class="hljs-built_in">ToLocalChecked</span>(),
args[<span class="hljs-number">0</span>]-><span class="hljs-built_in">ToString</span>(context).<span class="hljs-built_in">ToLocalChecked</span>())
.<span class="hljs-built_in">FromJust</span>();
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(obj);
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">Init</span><span class="hljs-params">(Local<Object> exports, Local<Object> <span class="hljs-keyword">module</span>)</span> </span>{
<span class="hljs-built_in">NODE_SET_METHOD</span>(<span class="hljs-keyword">module</span>, <span class="hljs-string">"exports"</span>, CreateObject);
}
<span class="hljs-built_in">NODE_MODULE</span>(NODE_GYP_MODULE_NAME, Init)
} <span class="hljs-comment">// namespace demo</span></code></pre>
<p>To test it in JavaScript:</p>
<pre><code class="language-js"><span class="hljs-comment">// test.js</span>
<span class="hljs-keyword">const</span> addon = <span class="hljs-built_in">require</span>(<span class="hljs-string">'./build/Release/addon'</span>);
<span class="hljs-keyword">const</span> obj1 = <span class="hljs-title function_">addon</span>(<span class="hljs-string">'hello'</span>);
<span class="hljs-keyword">const</span> obj2 = <span class="hljs-title function_">addon</span>(<span class="hljs-string">'world'</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(obj1.<span class="hljs-property">msg</span>, obj2.<span class="hljs-property">msg</span>);
<span class="hljs-comment">// Prints: 'hello world'</span></code></pre>
<h4>Function factory<span><a class="mark" href="#function-factory" id="function-factory">#</a></span><a aria-hidden="true" class="legacy" id="addons_function_factory"></a></h4>
<p>Another common scenario is creating JavaScript functions that wrap C++
functions and returning those back to JavaScript:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// addon.cc</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">using</span> v8::Context;
<span class="hljs-keyword">using</span> v8::Function;
<span class="hljs-keyword">using</span> v8::FunctionCallbackInfo;
<span class="hljs-keyword">using</span> v8::FunctionTemplate;
<span class="hljs-keyword">using</span> v8::Isolate;
<span class="hljs-keyword">using</span> v8::Local;
<span class="hljs-keyword">using</span> v8::Object;
<span class="hljs-keyword">using</span> v8::String;
<span class="hljs-keyword">using</span> v8::Value;
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">MyFunction</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(String::<span class="hljs-built_in">NewFromUtf8</span>(
isolate, <span class="hljs-string">"hello world"</span>).<span class="hljs-built_in">ToLocalChecked</span>());
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">CreateFunction</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
Local<Context> context = isolate-><span class="hljs-built_in">GetCurrentContext</span>();
Local<FunctionTemplate> tpl = FunctionTemplate::<span class="hljs-built_in">New</span>(isolate, MyFunction);
Local<Function> fn = tpl-><span class="hljs-built_in">GetFunction</span>(context).<span class="hljs-built_in">ToLocalChecked</span>();
<span class="hljs-comment">// omit this to make it anonymous</span>
fn-><span class="hljs-built_in">SetName</span>(String::<span class="hljs-built_in">NewFromUtf8</span>(
isolate, <span class="hljs-string">"theFunction"</span>).<span class="hljs-built_in">ToLocalChecked</span>());
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(fn);
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">Init</span><span class="hljs-params">(Local<Object> exports, Local<Object> <span class="hljs-keyword">module</span>)</span> </span>{
<span class="hljs-built_in">NODE_SET_METHOD</span>(<span class="hljs-keyword">module</span>, <span class="hljs-string">"exports"</span>, CreateFunction);
}
<span class="hljs-built_in">NODE_MODULE</span>(NODE_GYP_MODULE_NAME, Init)
} <span class="hljs-comment">// namespace demo</span></code></pre>
<p>To test:</p>
<pre><code class="language-js"><span class="hljs-comment">// test.js</span>
<span class="hljs-keyword">const</span> addon = <span class="hljs-built_in">require</span>(<span class="hljs-string">'./build/Release/addon'</span>);
<span class="hljs-keyword">const</span> fn = <span class="hljs-title function_">addon</span>();
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">fn</span>());
<span class="hljs-comment">// Prints: 'hello world'</span></code></pre>
<h4>Wrapping C++ objects<span><a class="mark" href="#wrapping-c-objects" id="wrapping-c-objects">#</a></span><a aria-hidden="true" class="legacy" id="addons_wrapping_c_objects"></a></h4>
<p>It is also possible to wrap C++ objects/classes in a way that allows new
instances to be created using the JavaScript <code>new</code> operator:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// addon.cc</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string">"myobject.h"</span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">using</span> v8::Local;
<span class="hljs-keyword">using</span> v8::Object;
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">InitAll</span><span class="hljs-params">(Local<Object> exports)</span> </span>{
MyObject::<span class="hljs-built_in">Init</span>(exports);
}
<span class="hljs-built_in">NODE_MODULE</span>(NODE_GYP_MODULE_NAME, InitAll)
} <span class="hljs-comment">// namespace demo</span></code></pre>
<p>Then, in <code>myobject.h</code>, the wrapper class inherits from <code>node::ObjectWrap</code>:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// myobject.h</span>
<span class="hljs-meta">#<span class="hljs-keyword">ifndef</span> MYOBJECT_H</span>
<span class="hljs-meta">#<span class="hljs-keyword">define</span> MYOBJECT_H</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node_object_wrap.h></span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">class</span> <span class="hljs-title class_">MyObject</span> : <span class="hljs-keyword">public</span> node::ObjectWrap {
<span class="hljs-keyword">public</span>:
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">Init</span><span class="hljs-params">(v8::Local<v8::Object> exports)</span></span>;
<span class="hljs-keyword">private</span>:
<span class="hljs-function"><span class="hljs-keyword">explicit</span> <span class="hljs-title">MyObject</span><span class="hljs-params">(<span class="hljs-type">double</span> value = <span class="hljs-number">0</span>)</span></span>;
~<span class="hljs-built_in">MyObject</span>();
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">New</span><span class="hljs-params">(<span class="hljs-type">const</span> v8::FunctionCallbackInfo<v8::Value>& args)</span></span>;
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">PlusOne</span><span class="hljs-params">(<span class="hljs-type">const</span> v8::FunctionCallbackInfo<v8::Value>& args)</span></span>;
<span class="hljs-type">double</span> value_;
};
} <span class="hljs-comment">// namespace demo</span>
<span class="hljs-meta">#<span class="hljs-keyword">endif</span></span></code></pre>
<p>In <code>myobject.cc</code>, implement the various methods that are to be exposed.
Below, the method <code>plusOne()</code> is exposed by adding it to the constructor's
prototype:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// myobject.cc</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string">"myobject.h"</span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">using</span> v8::Context;
<span class="hljs-keyword">using</span> v8::Function;
<span class="hljs-keyword">using</span> v8::FunctionCallbackInfo;
<span class="hljs-keyword">using</span> v8::FunctionTemplate;
<span class="hljs-keyword">using</span> v8::Isolate;
<span class="hljs-keyword">using</span> v8::Local;
<span class="hljs-keyword">using</span> v8::Number;
<span class="hljs-keyword">using</span> v8::Object;
<span class="hljs-keyword">using</span> v8::ObjectTemplate;
<span class="hljs-keyword">using</span> v8::String;
<span class="hljs-keyword">using</span> v8::Value;
MyObject::<span class="hljs-built_in">MyObject</span>(<span class="hljs-type">double</span> value) : <span class="hljs-built_in">value_</span>(value) {
}
MyObject::~<span class="hljs-built_in">MyObject</span>() {
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">MyObject::Init</span><span class="hljs-params">(Local<Object> exports)</span> </span>{
Isolate* isolate = exports-><span class="hljs-built_in">GetIsolate</span>();
Local<Context> context = isolate-><span class="hljs-built_in">GetCurrentContext</span>();
Local<ObjectTemplate> addon_data_tpl = ObjectTemplate::<span class="hljs-built_in">New</span>(isolate);
addon_data_tpl-><span class="hljs-built_in">SetInternalFieldCount</span>(<span class="hljs-number">1</span>); <span class="hljs-comment">// 1 field for the MyObject::New()</span>
Local<Object> addon_data =
addon_data_tpl-><span class="hljs-built_in">NewInstance</span>(context).<span class="hljs-built_in">ToLocalChecked</span>();
<span class="hljs-comment">// Prepare constructor template</span>
Local<FunctionTemplate> tpl = FunctionTemplate::<span class="hljs-built_in">New</span>(isolate, New, addon_data);
tpl-><span class="hljs-built_in">SetClassName</span>(String::<span class="hljs-built_in">NewFromUtf8</span>(isolate, <span class="hljs-string">"MyObject"</span>).<span class="hljs-built_in">ToLocalChecked</span>());
tpl-><span class="hljs-built_in">InstanceTemplate</span>()-><span class="hljs-built_in">SetInternalFieldCount</span>(<span class="hljs-number">1</span>);
<span class="hljs-comment">// Prototype</span>
<span class="hljs-built_in">NODE_SET_PROTOTYPE_METHOD</span>(tpl, <span class="hljs-string">"plusOne"</span>, PlusOne);
Local<Function> constructor = tpl-><span class="hljs-built_in">GetFunction</span>(context).<span class="hljs-built_in">ToLocalChecked</span>();
addon_data-><span class="hljs-built_in">SetInternalField</span>(<span class="hljs-number">0</span>, constructor);
exports-><span class="hljs-built_in">Set</span>(context, String::<span class="hljs-built_in">NewFromUtf8</span>(
isolate, <span class="hljs-string">"MyObject"</span>).<span class="hljs-built_in">ToLocalChecked</span>(),
constructor).<span class="hljs-built_in">FromJust</span>();
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">MyObject::New</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
Local<Context> context = isolate-><span class="hljs-built_in">GetCurrentContext</span>();
<span class="hljs-keyword">if</span> (args.<span class="hljs-built_in">IsConstructCall</span>()) {
<span class="hljs-comment">// Invoked as constructor: `new MyObject(...)`</span>
<span class="hljs-type">double</span> value = args[<span class="hljs-number">0</span>]-><span class="hljs-built_in">IsUndefined</span>() ?
<span class="hljs-number">0</span> : args[<span class="hljs-number">0</span>]-><span class="hljs-built_in">NumberValue</span>(context).<span class="hljs-built_in">FromMaybe</span>(<span class="hljs-number">0</span>);
MyObject* obj = <span class="hljs-keyword">new</span> <span class="hljs-built_in">MyObject</span>(value);
obj-><span class="hljs-built_in">Wrap</span>(args.<span class="hljs-built_in">This</span>());
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(args.<span class="hljs-built_in">This</span>());
} <span class="hljs-keyword">else</span> {
<span class="hljs-comment">// Invoked as plain function `MyObject(...)`, turn into construct call.</span>
<span class="hljs-type">const</span> <span class="hljs-type">int</span> argc = <span class="hljs-number">1</span>;
Local<Value> argv[argc] = { args[<span class="hljs-number">0</span>] };
Local<Function> cons =
args.<span class="hljs-built_in">Data</span>().<span class="hljs-built_in">As</span><Object>()-><span class="hljs-built_in">GetInternalField</span>(<span class="hljs-number">0</span>).<span class="hljs-built_in">As</span><Function>();
Local<Object> result =
cons-><span class="hljs-built_in">NewInstance</span>(context, argc, argv).<span class="hljs-built_in">ToLocalChecked</span>();
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(result);
}
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">MyObject::PlusOne</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
MyObject* obj = ObjectWrap::<span class="hljs-built_in">Unwrap</span><MyObject>(args.<span class="hljs-built_in">Holder</span>());
obj->value_ += <span class="hljs-number">1</span>;
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(Number::<span class="hljs-built_in">New</span>(isolate, obj->value_));
}
} <span class="hljs-comment">// namespace demo</span></code></pre>
<p>To build this example, the <code>myobject.cc</code> file must be added to the
<code>binding.gyp</code>:</p>
<pre><code class="language-json"><span class="hljs-punctuation">{</span>
<span class="hljs-attr">"targets"</span><span class="hljs-punctuation">:</span> <span class="hljs-punctuation">[</span>
<span class="hljs-punctuation">{</span>
<span class="hljs-attr">"target_name"</span><span class="hljs-punctuation">:</span> <span class="hljs-string">"addon"</span><span class="hljs-punctuation">,</span>
<span class="hljs-attr">"sources"</span><span class="hljs-punctuation">:</span> <span class="hljs-punctuation">[</span>
<span class="hljs-string">"addon.cc"</span><span class="hljs-punctuation">,</span>
<span class="hljs-string">"myobject.cc"</span>
<span class="hljs-punctuation">]</span>
<span class="hljs-punctuation">}</span>
<span class="hljs-punctuation">]</span>
<span class="hljs-punctuation">}</span></code></pre>
<p>Test it with:</p>
<pre><code class="language-js"><span class="hljs-comment">// test.js</span>
<span class="hljs-keyword">const</span> addon = <span class="hljs-built_in">require</span>(<span class="hljs-string">'./build/Release/addon'</span>);
<span class="hljs-keyword">const</span> obj = <span class="hljs-keyword">new</span> addon.<span class="hljs-title class_">MyObject</span>(<span class="hljs-number">10</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(obj.<span class="hljs-title function_">plusOne</span>());
<span class="hljs-comment">// Prints: 11</span>
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(obj.<span class="hljs-title function_">plusOne</span>());
<span class="hljs-comment">// Prints: 12</span>
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(obj.<span class="hljs-title function_">plusOne</span>());
<span class="hljs-comment">// Prints: 13</span></code></pre>
<p>The destructor for a wrapper object will run when the object is
garbage-collected. For destructor testing, there are command-line flags that
can be used to make it possible to force garbage collection. These flags are
provided by the underlying V8 JavaScript engine. They are subject to change
or removal at any time. They are not documented by Node.js or V8, and they
should never be used outside of testing.</p>
<p>During shutdown of the process or worker threads destructors are not called
by the JS engine. Therefore it's the responsibility of the user to track
these objects and ensure proper destruction to avoid resource leaks.</p>
<h4>Factory of wrapped objects<span><a class="mark" href="#factory-of-wrapped-objects" id="factory-of-wrapped-objects">#</a></span><a aria-hidden="true" class="legacy" id="addons_factory_of_wrapped_objects"></a></h4>
<p>Alternatively, it is possible to use a factory pattern to avoid explicitly
creating object instances using the JavaScript <code>new</code> operator:</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> obj = addon.<span class="hljs-title function_">createObject</span>();
<span class="hljs-comment">// instead of:</span>
<span class="hljs-comment">// const obj = new addon.Object();</span></code></pre>
<p>First, the <code>createObject()</code> method is implemented in <code>addon.cc</code>:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// addon.cc</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string">"myobject.h"</span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">using</span> v8::FunctionCallbackInfo;
<span class="hljs-keyword">using</span> v8::Isolate;
<span class="hljs-keyword">using</span> v8::Local;
<span class="hljs-keyword">using</span> v8::Object;
<span class="hljs-keyword">using</span> v8::String;
<span class="hljs-keyword">using</span> v8::Value;
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">CreateObject</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
MyObject::<span class="hljs-built_in">NewInstance</span>(args);
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">InitAll</span><span class="hljs-params">(Local<Object> exports, Local<Object> <span class="hljs-keyword">module</span>)</span> </span>{
MyObject::<span class="hljs-built_in">Init</span>(exports-><span class="hljs-built_in">GetIsolate</span>());
<span class="hljs-built_in">NODE_SET_METHOD</span>(<span class="hljs-keyword">module</span>, <span class="hljs-string">"exports"</span>, CreateObject);
}
<span class="hljs-built_in">NODE_MODULE</span>(NODE_GYP_MODULE_NAME, InitAll)
} <span class="hljs-comment">// namespace demo</span></code></pre>
<p>In <code>myobject.h</code>, the static method <code>NewInstance()</code> is added to handle
instantiating the object. This method takes the place of using <code>new</code> in
JavaScript:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// myobject.h</span>
<span class="hljs-meta">#<span class="hljs-keyword">ifndef</span> MYOBJECT_H</span>
<span class="hljs-meta">#<span class="hljs-keyword">define</span> MYOBJECT_H</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node_object_wrap.h></span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">class</span> <span class="hljs-title class_">MyObject</span> : <span class="hljs-keyword">public</span> node::ObjectWrap {
<span class="hljs-keyword">public</span>:
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">Init</span><span class="hljs-params">(v8::Isolate* isolate)</span></span>;
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">NewInstance</span><span class="hljs-params">(<span class="hljs-type">const</span> v8::FunctionCallbackInfo<v8::Value>& args)</span></span>;
<span class="hljs-keyword">private</span>:
<span class="hljs-function"><span class="hljs-keyword">explicit</span> <span class="hljs-title">MyObject</span><span class="hljs-params">(<span class="hljs-type">double</span> value = <span class="hljs-number">0</span>)</span></span>;
~<span class="hljs-built_in">MyObject</span>();
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">New</span><span class="hljs-params">(<span class="hljs-type">const</span> v8::FunctionCallbackInfo<v8::Value>& args)</span></span>;
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">PlusOne</span><span class="hljs-params">(<span class="hljs-type">const</span> v8::FunctionCallbackInfo<v8::Value>& args)</span></span>;
<span class="hljs-type">static</span> v8::Global<v8::Function> constructor;
<span class="hljs-type">double</span> value_;
};
} <span class="hljs-comment">// namespace demo</span>
<span class="hljs-meta">#<span class="hljs-keyword">endif</span></span></code></pre>
<p>The implementation in <code>myobject.cc</code> is similar to the previous example:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// myobject.cc</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string">"myobject.h"</span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">using</span> node::AddEnvironmentCleanupHook;
<span class="hljs-keyword">using</span> v8::Context;
<span class="hljs-keyword">using</span> v8::Function;
<span class="hljs-keyword">using</span> v8::FunctionCallbackInfo;
<span class="hljs-keyword">using</span> v8::FunctionTemplate;
<span class="hljs-keyword">using</span> v8::Global;
<span class="hljs-keyword">using</span> v8::Isolate;
<span class="hljs-keyword">using</span> v8::Local;
<span class="hljs-keyword">using</span> v8::Number;
<span class="hljs-keyword">using</span> v8::Object;
<span class="hljs-keyword">using</span> v8::String;
<span class="hljs-keyword">using</span> v8::Value;
<span class="hljs-comment">// Warning! This is not thread-safe, this addon cannot be used for worker</span>
<span class="hljs-comment">// threads.</span>
Global<Function> MyObject::constructor;
MyObject::<span class="hljs-built_in">MyObject</span>(<span class="hljs-type">double</span> value) : <span class="hljs-built_in">value_</span>(value) {
}
MyObject::~<span class="hljs-built_in">MyObject</span>() {
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">MyObject::Init</span><span class="hljs-params">(Isolate* isolate)</span> </span>{
<span class="hljs-comment">// Prepare constructor template</span>
Local<FunctionTemplate> tpl = FunctionTemplate::<span class="hljs-built_in">New</span>(isolate, New);
tpl-><span class="hljs-built_in">SetClassName</span>(String::<span class="hljs-built_in">NewFromUtf8</span>(isolate, <span class="hljs-string">"MyObject"</span>).<span class="hljs-built_in">ToLocalChecked</span>());
tpl-><span class="hljs-built_in">InstanceTemplate</span>()-><span class="hljs-built_in">SetInternalFieldCount</span>(<span class="hljs-number">1</span>);
<span class="hljs-comment">// Prototype</span>
<span class="hljs-built_in">NODE_SET_PROTOTYPE_METHOD</span>(tpl, <span class="hljs-string">"plusOne"</span>, PlusOne);
Local<Context> context = isolate-><span class="hljs-built_in">GetCurrentContext</span>();
constructor.<span class="hljs-built_in">Reset</span>(isolate, tpl-><span class="hljs-built_in">GetFunction</span>(context).<span class="hljs-built_in">ToLocalChecked</span>());
<span class="hljs-built_in">AddEnvironmentCleanupHook</span>(isolate, [](<span class="hljs-type">void</span>*) {
constructor.<span class="hljs-built_in">Reset</span>();
}, <span class="hljs-literal">nullptr</span>);
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">MyObject::New</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
Local<Context> context = isolate-><span class="hljs-built_in">GetCurrentContext</span>();
<span class="hljs-keyword">if</span> (args.<span class="hljs-built_in">IsConstructCall</span>()) {
<span class="hljs-comment">// Invoked as constructor: `new MyObject(...)`</span>
<span class="hljs-type">double</span> value = args[<span class="hljs-number">0</span>]-><span class="hljs-built_in">IsUndefined</span>() ?
<span class="hljs-number">0</span> : args[<span class="hljs-number">0</span>]-><span class="hljs-built_in">NumberValue</span>(context).<span class="hljs-built_in">FromMaybe</span>(<span class="hljs-number">0</span>);
MyObject* obj = <span class="hljs-keyword">new</span> <span class="hljs-built_in">MyObject</span>(value);
obj-><span class="hljs-built_in">Wrap</span>(args.<span class="hljs-built_in">This</span>());
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(args.<span class="hljs-built_in">This</span>());
} <span class="hljs-keyword">else</span> {
<span class="hljs-comment">// Invoked as plain function `MyObject(...)`, turn into construct call.</span>
<span class="hljs-type">const</span> <span class="hljs-type">int</span> argc = <span class="hljs-number">1</span>;
Local<Value> argv[argc] = { args[<span class="hljs-number">0</span>] };
Local<Function> cons = Local<Function>::<span class="hljs-built_in">New</span>(isolate, constructor);
Local<Object> instance =
cons-><span class="hljs-built_in">NewInstance</span>(context, argc, argv).<span class="hljs-built_in">ToLocalChecked</span>();
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(instance);
}
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">MyObject::NewInstance</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
<span class="hljs-type">const</span> <span class="hljs-type">unsigned</span> argc = <span class="hljs-number">1</span>;
Local<Value> argv[argc] = { args[<span class="hljs-number">0</span>] };
Local<Function> cons = Local<Function>::<span class="hljs-built_in">New</span>(isolate, constructor);
Local<Context> context = isolate-><span class="hljs-built_in">GetCurrentContext</span>();
Local<Object> instance =
cons-><span class="hljs-built_in">NewInstance</span>(context, argc, argv).<span class="hljs-built_in">ToLocalChecked</span>();
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(instance);
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">MyObject::PlusOne</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
MyObject* obj = ObjectWrap::<span class="hljs-built_in">Unwrap</span><MyObject>(args.<span class="hljs-built_in">Holder</span>());
obj->value_ += <span class="hljs-number">1</span>;
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(Number::<span class="hljs-built_in">New</span>(isolate, obj->value_));
}
} <span class="hljs-comment">// namespace demo</span></code></pre>
<p>Once again, to build this example, the <code>myobject.cc</code> file must be added to the
<code>binding.gyp</code>:</p>
<pre><code class="language-json"><span class="hljs-punctuation">{</span>
<span class="hljs-attr">"targets"</span><span class="hljs-punctuation">:</span> <span class="hljs-punctuation">[</span>
<span class="hljs-punctuation">{</span>
<span class="hljs-attr">"target_name"</span><span class="hljs-punctuation">:</span> <span class="hljs-string">"addon"</span><span class="hljs-punctuation">,</span>
<span class="hljs-attr">"sources"</span><span class="hljs-punctuation">:</span> <span class="hljs-punctuation">[</span>
<span class="hljs-string">"addon.cc"</span><span class="hljs-punctuation">,</span>
<span class="hljs-string">"myobject.cc"</span>
<span class="hljs-punctuation">]</span>
<span class="hljs-punctuation">}</span>
<span class="hljs-punctuation">]</span>
<span class="hljs-punctuation">}</span></code></pre>
<p>Test it with:</p>
<pre><code class="language-js"><span class="hljs-comment">// test.js</span>
<span class="hljs-keyword">const</span> createObject = <span class="hljs-built_in">require</span>(<span class="hljs-string">'./build/Release/addon'</span>);
<span class="hljs-keyword">const</span> obj = <span class="hljs-title function_">createObject</span>(<span class="hljs-number">10</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(obj.<span class="hljs-title function_">plusOne</span>());
<span class="hljs-comment">// Prints: 11</span>
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(obj.<span class="hljs-title function_">plusOne</span>());
<span class="hljs-comment">// Prints: 12</span>
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(obj.<span class="hljs-title function_">plusOne</span>());
<span class="hljs-comment">// Prints: 13</span>
<span class="hljs-keyword">const</span> obj2 = <span class="hljs-title function_">createObject</span>(<span class="hljs-number">20</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(obj2.<span class="hljs-title function_">plusOne</span>());
<span class="hljs-comment">// Prints: 21</span>
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(obj2.<span class="hljs-title function_">plusOne</span>());
<span class="hljs-comment">// Prints: 22</span>
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(obj2.<span class="hljs-title function_">plusOne</span>());
<span class="hljs-comment">// Prints: 23</span></code></pre>
<h4>Passing wrapped objects around<span><a class="mark" href="#passing-wrapped-objects-around" id="passing-wrapped-objects-around">#</a></span><a aria-hidden="true" class="legacy" id="addons_passing_wrapped_objects_around"></a></h4>
<p>In addition to wrapping and returning C++ objects, it is possible to pass
wrapped objects around by unwrapping them with the Node.js helper function
<code>node::ObjectWrap::Unwrap</code>. The following examples shows a function <code>add()</code>
that can take two <code>MyObject</code> objects as input arguments:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// addon.cc</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node_object_wrap.h></span></span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string">"myobject.h"</span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">using</span> v8::Context;
<span class="hljs-keyword">using</span> v8::FunctionCallbackInfo;
<span class="hljs-keyword">using</span> v8::Isolate;
<span class="hljs-keyword">using</span> v8::Local;
<span class="hljs-keyword">using</span> v8::Number;
<span class="hljs-keyword">using</span> v8::Object;
<span class="hljs-keyword">using</span> v8::String;
<span class="hljs-keyword">using</span> v8::Value;
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">CreateObject</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
MyObject::<span class="hljs-built_in">NewInstance</span>(args);
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">Add</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
Local<Context> context = isolate-><span class="hljs-built_in">GetCurrentContext</span>();
MyObject* obj1 = node::ObjectWrap::<span class="hljs-built_in">Unwrap</span><MyObject>(
args[<span class="hljs-number">0</span>]-><span class="hljs-built_in">ToObject</span>(context).<span class="hljs-built_in">ToLocalChecked</span>());
MyObject* obj2 = node::ObjectWrap::<span class="hljs-built_in">Unwrap</span><MyObject>(
args[<span class="hljs-number">1</span>]-><span class="hljs-built_in">ToObject</span>(context).<span class="hljs-built_in">ToLocalChecked</span>());
<span class="hljs-type">double</span> sum = obj1-><span class="hljs-built_in">value</span>() + obj2-><span class="hljs-built_in">value</span>();
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(Number::<span class="hljs-built_in">New</span>(isolate, sum));
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">InitAll</span><span class="hljs-params">(Local<Object> exports)</span> </span>{
MyObject::<span class="hljs-built_in">Init</span>(exports-><span class="hljs-built_in">GetIsolate</span>());
<span class="hljs-built_in">NODE_SET_METHOD</span>(exports, <span class="hljs-string">"createObject"</span>, CreateObject);
<span class="hljs-built_in">NODE_SET_METHOD</span>(exports, <span class="hljs-string">"add"</span>, Add);
}
<span class="hljs-built_in">NODE_MODULE</span>(NODE_GYP_MODULE_NAME, InitAll)
} <span class="hljs-comment">// namespace demo</span></code></pre>
<p>In <code>myobject.h</code>, a new public method is added to allow access to private values
after unwrapping the object.</p>
<pre><code class="language-cpp"><span class="hljs-comment">// myobject.h</span>
<span class="hljs-meta">#<span class="hljs-keyword">ifndef</span> MYOBJECT_H</span>
<span class="hljs-meta">#<span class="hljs-keyword">define</span> MYOBJECT_H</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node_object_wrap.h></span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">class</span> <span class="hljs-title class_">MyObject</span> : <span class="hljs-keyword">public</span> node::ObjectWrap {
<span class="hljs-keyword">public</span>:
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">Init</span><span class="hljs-params">(v8::Isolate* isolate)</span></span>;
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">NewInstance</span><span class="hljs-params">(<span class="hljs-type">const</span> v8::FunctionCallbackInfo<v8::Value>& args)</span></span>;
<span class="hljs-function"><span class="hljs-keyword">inline</span> <span class="hljs-type">double</span> <span class="hljs-title">value</span><span class="hljs-params">()</span> <span class="hljs-type">const</span> </span>{ <span class="hljs-keyword">return</span> value_; }
<span class="hljs-keyword">private</span>:
<span class="hljs-function"><span class="hljs-keyword">explicit</span> <span class="hljs-title">MyObject</span><span class="hljs-params">(<span class="hljs-type">double</span> value = <span class="hljs-number">0</span>)</span></span>;
~<span class="hljs-built_in">MyObject</span>();
<span class="hljs-function"><span class="hljs-type">static</span> <span class="hljs-type">void</span> <span class="hljs-title">New</span><span class="hljs-params">(<span class="hljs-type">const</span> v8::FunctionCallbackInfo<v8::Value>& args)</span></span>;
<span class="hljs-type">static</span> v8::Global<v8::Function> constructor;
<span class="hljs-type">double</span> value_;
};
} <span class="hljs-comment">// namespace demo</span>
<span class="hljs-meta">#<span class="hljs-keyword">endif</span></span></code></pre>
<p>The implementation of <code>myobject.cc</code> is similar to before:</p>
<pre><code class="language-cpp"><span class="hljs-comment">// myobject.cc</span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><node.h></span></span>
<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string">"myobject.h"</span></span>
<span class="hljs-keyword">namespace</span> demo {
<span class="hljs-keyword">using</span> node::AddEnvironmentCleanupHook;
<span class="hljs-keyword">using</span> v8::Context;
<span class="hljs-keyword">using</span> v8::Function;
<span class="hljs-keyword">using</span> v8::FunctionCallbackInfo;
<span class="hljs-keyword">using</span> v8::FunctionTemplate;
<span class="hljs-keyword">using</span> v8::Global;
<span class="hljs-keyword">using</span> v8::Isolate;
<span class="hljs-keyword">using</span> v8::Local;
<span class="hljs-keyword">using</span> v8::Object;
<span class="hljs-keyword">using</span> v8::String;
<span class="hljs-keyword">using</span> v8::Value;
<span class="hljs-comment">// Warning! This is not thread-safe, this addon cannot be used for worker</span>
<span class="hljs-comment">// threads.</span>
Global<Function> MyObject::constructor;
MyObject::<span class="hljs-built_in">MyObject</span>(<span class="hljs-type">double</span> value) : <span class="hljs-built_in">value_</span>(value) {
}
MyObject::~<span class="hljs-built_in">MyObject</span>() {
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">MyObject::Init</span><span class="hljs-params">(Isolate* isolate)</span> </span>{
<span class="hljs-comment">// Prepare constructor template</span>
Local<FunctionTemplate> tpl = FunctionTemplate::<span class="hljs-built_in">New</span>(isolate, New);
tpl-><span class="hljs-built_in">SetClassName</span>(String::<span class="hljs-built_in">NewFromUtf8</span>(isolate, <span class="hljs-string">"MyObject"</span>).<span class="hljs-built_in">ToLocalChecked</span>());
tpl-><span class="hljs-built_in">InstanceTemplate</span>()-><span class="hljs-built_in">SetInternalFieldCount</span>(<span class="hljs-number">1</span>);
Local<Context> context = isolate-><span class="hljs-built_in">GetCurrentContext</span>();
constructor.<span class="hljs-built_in">Reset</span>(isolate, tpl-><span class="hljs-built_in">GetFunction</span>(context).<span class="hljs-built_in">ToLocalChecked</span>());
<span class="hljs-built_in">AddEnvironmentCleanupHook</span>(isolate, [](<span class="hljs-type">void</span>*) {
constructor.<span class="hljs-built_in">Reset</span>();
}, <span class="hljs-literal">nullptr</span>);
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">MyObject::New</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
Local<Context> context = isolate-><span class="hljs-built_in">GetCurrentContext</span>();
<span class="hljs-keyword">if</span> (args.<span class="hljs-built_in">IsConstructCall</span>()) {
<span class="hljs-comment">// Invoked as constructor: `new MyObject(...)`</span>
<span class="hljs-type">double</span> value = args[<span class="hljs-number">0</span>]-><span class="hljs-built_in">IsUndefined</span>() ?
<span class="hljs-number">0</span> : args[<span class="hljs-number">0</span>]-><span class="hljs-built_in">NumberValue</span>(context).<span class="hljs-built_in">FromMaybe</span>(<span class="hljs-number">0</span>);
MyObject* obj = <span class="hljs-keyword">new</span> <span class="hljs-built_in">MyObject</span>(value);
obj-><span class="hljs-built_in">Wrap</span>(args.<span class="hljs-built_in">This</span>());
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(args.<span class="hljs-built_in">This</span>());
} <span class="hljs-keyword">else</span> {
<span class="hljs-comment">// Invoked as plain function `MyObject(...)`, turn into construct call.</span>
<span class="hljs-type">const</span> <span class="hljs-type">int</span> argc = <span class="hljs-number">1</span>;
Local<Value> argv[argc] = { args[<span class="hljs-number">0</span>] };
Local<Function> cons = Local<Function>::<span class="hljs-built_in">New</span>(isolate, constructor);
Local<Object> instance =
cons-><span class="hljs-built_in">NewInstance</span>(context, argc, argv).<span class="hljs-built_in">ToLocalChecked</span>();
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(instance);
}
}
<span class="hljs-function"><span class="hljs-type">void</span> <span class="hljs-title">MyObject::NewInstance</span><span class="hljs-params">(<span class="hljs-type">const</span> FunctionCallbackInfo<Value>& args)</span> </span>{
Isolate* isolate = args.<span class="hljs-built_in">GetIsolate</span>();
<span class="hljs-type">const</span> <span class="hljs-type">unsigned</span> argc = <span class="hljs-number">1</span>;
Local<Value> argv[argc] = { args[<span class="hljs-number">0</span>] };
Local<Function> cons = Local<Function>::<span class="hljs-built_in">New</span>(isolate, constructor);
Local<Context> context = isolate-><span class="hljs-built_in">GetCurrentContext</span>();
Local<Object> instance =
cons-><span class="hljs-built_in">NewInstance</span>(context, argc, argv).<span class="hljs-built_in">ToLocalChecked</span>();
args.<span class="hljs-built_in">GetReturnValue</span>().<span class="hljs-built_in">Set</span>(instance);
}
} <span class="hljs-comment">// namespace demo</span></code></pre>
<p>Test it with:</p>
<pre><code class="language-js"><span class="hljs-comment">// test.js</span>
<span class="hljs-keyword">const</span> addon = <span class="hljs-built_in">require</span>(<span class="hljs-string">'./build/Release/addon'</span>);
<span class="hljs-keyword">const</span> obj1 = addon.<span class="hljs-title function_">createObject</span>(<span class="hljs-number">10</span>);
<span class="hljs-keyword">const</span> obj2 = addon.<span class="hljs-title function_">createObject</span>(<span class="hljs-number">20</span>);
<span class="hljs-keyword">const</span> result = addon.<span class="hljs-title function_">add</span>(obj1, obj2);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(result);
<span class="hljs-comment">// Prints: 30</span></code></pre></section>
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