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/**
* @fileoverview Flag expressions in statement position that do not side effect
* @author Michael Ficarra
*/
"use strict";
//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------
/**
* Returns `true`.
* @returns {boolean} `true`.
*/
function alwaysTrue() {
return true;
}
/**
* Returns `false`.
* @returns {boolean} `false`.
*/
function alwaysFalse() {
return false;
}
/** @type {import('../shared/types').Rule} */
module.exports = {
meta: {
type: "suggestion",
docs: {
description: "disallow unused expressions",
recommended: false,
url: "https://eslint.org/docs/rules/no-unused-expressions"
},
schema: [
{
type: "object",
properties: {
allowShortCircuit: {
type: "boolean",
default: false
},
allowTernary: {
type: "boolean",
default: false
},
allowTaggedTemplates: {
type: "boolean",
default: false
},
enforceForJSX: {
type: "boolean",
default: false
}
},
additionalProperties: false
}
],
messages: {
unusedExpression: "Expected an assignment or function call and instead saw an expression."
}
},
create(context) {
const config = context.options[0] || {},
allowShortCircuit = config.allowShortCircuit || false,
allowTernary = config.allowTernary || false,
allowTaggedTemplates = config.allowTaggedTemplates || false,
enforceForJSX = config.enforceForJSX || false;
/**
* Has AST suggesting a directive.
* @param {ASTNode} node any node
* @returns {boolean} whether the given node structurally represents a directive
*/
function looksLikeDirective(node) {
return node.type === "ExpressionStatement" &&
node.expression.type === "Literal" && typeof node.expression.value === "string";
}
/**
* Gets the leading sequence of members in a list that pass the predicate.
* @param {Function} predicate ([a] -> Boolean) the function used to make the determination
* @param {a[]} list the input list
* @returns {a[]} the leading sequence of members in the given list that pass the given predicate
*/
function takeWhile(predicate, list) {
for (let i = 0; i < list.length; ++i) {
if (!predicate(list[i])) {
return list.slice(0, i);
}
}
return list.slice();
}
/**
* Gets leading directives nodes in a Node body.
* @param {ASTNode} node a Program or BlockStatement node
* @returns {ASTNode[]} the leading sequence of directive nodes in the given node's body
*/
function directives(node) {
return takeWhile(looksLikeDirective, node.body);
}
/**
* Detect if a Node is a directive.
* @param {ASTNode} node any node
* @param {ASTNode[]} ancestors the given node's ancestors
* @returns {boolean} whether the given node is considered a directive in its current position
*/
function isDirective(node, ancestors) {
const parent = ancestors[ancestors.length - 1],
grandparent = ancestors[ancestors.length - 2];
/**
* https://tc39.es/ecma262/#directive-prologue
*
* Only `FunctionBody`, `ScriptBody` and `ModuleBody` can have directive prologue.
* Class static blocks do not have directive prologue.
*/
return (parent.type === "Program" || parent.type === "BlockStatement" &&
(/Function/u.test(grandparent.type))) &&
directives(parent).indexOf(node) >= 0;
}
/**
* The member functions return `true` if the type has no side-effects.
* Unknown nodes are handled as `false`, then this rule ignores those.
*/
const Checker = Object.assign(Object.create(null), {
isDisallowed(node) {
return (Checker[node.type] || alwaysFalse)(node);
},
ArrayExpression: alwaysTrue,
ArrowFunctionExpression: alwaysTrue,
BinaryExpression: alwaysTrue,
ChainExpression(node) {
return Checker.isDisallowed(node.expression);
},
ClassExpression: alwaysTrue,
ConditionalExpression(node) {
if (allowTernary) {
return Checker.isDisallowed(node.consequent) || Checker.isDisallowed(node.alternate);
}
return true;
},
FunctionExpression: alwaysTrue,
Identifier: alwaysTrue,
JSXElement() {
return enforceForJSX;
},
JSXFragment() {
return enforceForJSX;
},
Literal: alwaysTrue,
LogicalExpression(node) {
if (allowShortCircuit) {
return Checker.isDisallowed(node.right);
}
return true;
},
MemberExpression: alwaysTrue,
MetaProperty: alwaysTrue,
ObjectExpression: alwaysTrue,
SequenceExpression: alwaysTrue,
TaggedTemplateExpression() {
return !allowTaggedTemplates;
},
TemplateLiteral: alwaysTrue,
ThisExpression: alwaysTrue,
UnaryExpression(node) {
return node.operator !== "void" && node.operator !== "delete";
}
});
return {
ExpressionStatement(node) {
if (Checker.isDisallowed(node.expression) && !isDirective(node, context.getAncestors())) {
context.report({ node, messageId: "unusedExpression" });
}
}
};
}
};
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