'use strict';
/*
* relax complexity and max-statements eslint rules in order to preserve the imported
* core eslint `prefer-arrow-callback` rule code so that future updates to that code
* can be more easily applied here.
*/
/* eslint "complexity": [ "error", 18 ], "max-statements": [ "error", 15 ] */
/**
* @fileoverview A rule to suggest using arrow functions as callbacks.
* @author Toru Nagashima (core eslint rule)
* @author Michael Fields (mocha-aware rule modifications)
*/
const createAstUtils = require('../util/ast');
// ------------------------------------------------------------------------------
// Helpers
// ------------------------------------------------------------------------------
/**
* Checks whether or not a given variable is a function name.
* @param {eslint-scope.Variable} variable - A variable to check.
* @returns {boolean} `true` if the variable is a function name.
*/
function isFunctionName(variable) {
return variable && variable.defs[0].type === 'FunctionName';
}
/**
* Gets the variable object of `arguments` which is defined implicitly.
* @param {eslint-scope.Scope} scope - A scope to get.
* @returns {eslint-scope.Variable} The found variable object.
*/
function getVariableOfArguments(scope) {
const variables = scope.variables;
let variableObject = null;
for (let i = 0; i < variables.length; i += 1) {
const variable = variables[i];
/*
* If there was a parameter which is named "arguments", the
* implicit "arguments" is not defined.
* So does fast return with null.
*/
if (variable.name === 'arguments' && variable.identifiers.length === 0) {
variableObject = variable;
break;
}
}
return variableObject;
}
/**
* Does the node property indicate a `bind` identifier
* @param {object} property - the node property.
* @returns {boolean}
*/
function propertyIndicatesBind(property) {
return !property.computed &&
property.type === 'Identifier' &&
property.name === 'bind';
}
/**
* Is the node a `.bind(this)` node?
* @param {ASTNode} node - the node property.
* @returns {boolean}
*/
function isBindThis(node, currentNode) {
return node.object === currentNode &&
propertyIndicatesBind(node.property) &&
node.parent.type === 'CallExpression' &&
node.parent.callee === node;
}
/**
* Checks whether a simple list of parameters contains any duplicates. This does not handle complex
* parameter lists (e.g. with destructuring), since complex parameter lists are a SyntaxError with duplicate
* parameter names anyway. Instead, it always returns `false` for complex parameter lists.
* @param {ASTNode[]} paramsList The list of parameters for a function
* @returns {boolean} `true` if the list of parameters contains any duplicates
*/
function hasDuplicateParams(paramsList) {
return paramsList.every((param) => param.type === 'Identifier') &&
paramsList.length !== new Set(paramsList.map((param) => param.name)).size;
}
// ------------------------------------------------------------------------------
// Rule Definition
// ------------------------------------------------------------------------------
module.exports = {
meta: {
type: 'suggestion',
docs: {
description: 'Require using arrow functions for callbacks',
category: 'ECMAScript 6',
recommended: false,
url: 'https://eslint.org/docs/rules/prefer-arrow-callback'
},
schema: [
{
type: 'object',
properties: {
allowNamedFunctions: {
type: 'boolean'
},
allowUnboundThis: {
type: 'boolean'
}
},
additionalProperties: false
}
],
fixable: 'code'
},
create(context) {
const astUtils = createAstUtils(context.settings);
const options = context.options[0] || {};
// allowUnboundThis defaults to true
const allowUnboundThis = options.allowUnboundThis !== false;
const allowNamedFunctions = options.allowNamedFunctions;
const sourceCode = context.getSourceCode();
/**
* Checkes whether or not a given node is a callback.
* @param {ASTNode} node - A node to check.
* @param {object} context - The eslint context.
* @returns {Object}
* {boolean} retv.isCallback - `true` if the node is a callback.
* {boolean} retv.isMochaCallback - `true` if the node is an argument to a mocha function.
* {boolean} retv.isLexicalThis - `true` if the node is with `.bind(this)`.
*/
function getCallbackInfo(node) {
const retv = { isCallback: false, isLexicalThis: false };
let searchComplete = false;
let currentNode = node;
let parent = node.parent;
while (currentNode && !searchComplete) {
switch (parent.type) {
// Checks parents recursively.
case 'LogicalExpression':
case 'ConditionalExpression':
break;
// Checks whether the parent node is `.bind(this)` call.
case 'MemberExpression':
if (isBindThis(parent, currentNode)) {
retv.isLexicalThis =
parent.parent.arguments.length === 1 &&
parent.parent.arguments[0].type === 'ThisExpression';
parent = parent.parent;
} else {
searchComplete = true;
}
break;
// Checks whether the node is a callback.
case 'CallExpression':
case 'NewExpression':
if (parent.callee !== currentNode) {
retv.isCallback = true;
}
// Checks whether the node is a mocha function callback.
if (retv.isCallback && astUtils.isMochaFunctionCall(parent, context.getScope())) {
retv.isMochaCallback = true;
}
searchComplete = true;
break;
default:
searchComplete = true;
}
if (!searchComplete) {
currentNode = parent;
parent = parent.parent;
}
}
return retv;
}
/*
* {Array<{this: boolean, meta: boolean}>}
* - this - A flag which shows there are one or more ThisExpression.
* - meta - A flag which shows there are one or more MetaProperty.
*/
let stack = [];
/**
* Pushes new function scope with all `false` flags.
* @returns {void}
*/
function enterScope() {
stack.push({ this: false, meta: false });
}
/**
* Pops a function scope from the stack.
* @returns {{this: boolean, meta: boolean}} The information of the last scope.
*/
function exitScope() {
return stack.pop();
}
return {
// Reset internal state.
Program() {
stack = [];
},
// If there are below, it cannot replace with arrow functions merely.
ThisExpression() {
const info = stack[stack.length - 1];
if (info) {
info.this = true;
}
},
MetaProperty() {
const info = stack[stack.length - 1];
info.meta = true;
},
// To skip nested scopes.
FunctionDeclaration: enterScope,
'FunctionDeclaration:exit': exitScope,
// Main.
FunctionExpression: enterScope,
'FunctionExpression:exit'(node) {
const scopeInfo = exitScope();
// Skip named function expressions
if (allowNamedFunctions && node.id && node.id.name) {
return;
}
// Skip generators.
if (node.generator) {
return;
}
// Skip recursive functions.
const nameVar = context.getDeclaredVariables(node)[0];
if (isFunctionName(nameVar) && nameVar.references.length > 0) {
return;
}
// Skip if it's using arguments.
const variable = getVariableOfArguments(context.getScope());
if (variable && variable.references.length > 0) {
return;
}
// Reports if it's a callback which can replace with arrows.
const callbackInfo = getCallbackInfo(node, context);
if (callbackInfo.isCallback &&
(!allowUnboundThis || !scopeInfo.this || callbackInfo.isLexicalThis) &&
!scopeInfo.meta &&
!callbackInfo.isMochaCallback
) {
context.report({
node,
message: 'Unexpected function expression.',
fix(fixer) {
if (!callbackInfo.isLexicalThis && scopeInfo.this || hasDuplicateParams(node.params)) {
/*
* If the callback function does not have .bind(this) and contains a reference to
* `this`, there is no way to determine what `this` should be, so don't perform any
* fixes. If the callback function has duplicates in its list of parameters (possible
* in sloppy mode), don't replace it with an arrow function, because this is a
* SyntaxError with arrow functions.
*/
return null;
}
const paramsLeftParen = node.params.length ?
sourceCode.getTokenBefore(node.params[0]) :
sourceCode.getTokenBefore(node.body, 1);
const paramsRightParen = sourceCode.getTokenBefore(node.body);
const asyncKeyword = node.async ? 'async ' : '';
const paramsFullText = sourceCode.text.slice(
paramsLeftParen.range[0], paramsRightParen.range[1]
);
const arrowFunctionText =
`${asyncKeyword}${paramsFullText} => ${sourceCode.getText(node.body)}`;
/*
* If the callback function has `.bind(this)`, replace it with an arrow function and remove
* the binding. Otherwise, just replace the arrow function itself.
*/
const replacedNode = callbackInfo.isLexicalThis ? node.parent.parent : node;
/*
* If the replaced node is part of a BinaryExpression, LogicalExpression, or
* MemberExpression, then the arrow function needs to be parenthesized, because
* `foo || () => {}` is invalid syntax even though `foo || function() {}` is valid.
*/
const needsParens = replacedNode.parent.type !== 'CallExpression' &&
replacedNode.parent.type !== 'ConditionalExpression';
const replacementText = needsParens ? `(${arrowFunctionText})` : arrowFunctionText;
return fixer.replaceText(replacedNode, replacementText);
}
});
}
}
};
}
};