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/**
* @fileoverview Flag expressions in statement position that do not side effect
* @author Michael Ficarra
*/
"use strict";
//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------
module.exports = {
meta: {
type: "suggestion",
docs: {
description: "disallow unused expressions",
category: "Best Practices",
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
}
},
additionalProperties: false
}
]
},
create(context) {
const config = context.options[0] || {},
allowShortCircuit = config.allowShortCircuit || false,
allowTernary = config.allowTernary || false,
allowTaggedTemplates = config.allowTaggedTemplates || false;
/**
* @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";
}
/**
* @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();
}
/**
* @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);
}
/**
* @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];
return (parent.type === "Program" || parent.type === "BlockStatement" &&
(/Function/u.test(grandparent.type))) &&
directives(parent).indexOf(node) >= 0;
}
/**
* Determines whether or not a given node is a valid expression. Recurses on short circuit eval and ternary nodes if enabled by flags.
* @param {ASTNode} node - any node
* @returns {boolean} whether the given node is a valid expression
*/
function isValidExpression(node) {
if (allowTernary) {
// Recursive check for ternary and logical expressions
if (node.type === "ConditionalExpression") {
return isValidExpression(node.consequent) && isValidExpression(node.alternate);
}
}
if (allowShortCircuit) {
if (node.type === "LogicalExpression") {
return isValidExpression(node.right);
}
}
if (allowTaggedTemplates && node.type === "TaggedTemplateExpression") {
return true;
}
return /^(?:Assignment|Call|New|Update|Yield|Await)Expression$/u.test(node.type) ||
(node.type === "UnaryExpression" && ["delete", "void"].indexOf(node.operator) >= 0);
}
return {
ExpressionStatement(node) {
if (!isValidExpression(node.expression) && !isDirective(node, context.getAncestors())) {
context.report({ node, message: "Expected an assignment or function call and instead saw an expression." });
}
}
};
}
};