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/**
* @fileoverview A rule to suggest using arrow functions as callbacks.
* @author Toru Nagashima
*/
"use strict";
//------------------------------------------------------------------------------
// 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";
}
/**
* Checks whether or not a given MetaProperty node equals to a given value.
* @param {ASTNode} node A MetaProperty node to check.
* @param {string} metaName The name of `MetaProperty.meta`.
* @param {string} propertyName The name of `MetaProperty.property`.
* @returns {boolean} `true` if the node is the specific value.
*/
function checkMetaProperty(node, metaName, propertyName) {
return node.meta.name === metaName && node.property.name === propertyName;
}
/**
* 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;
for (let i = 0; i < variables.length; ++i) {
const variable = variables[i];
if (variable.name === "arguments") {
/*
* If there was a parameter which is named "arguments", the
* implicit "arguments" is not defined.
* So does fast return with null.
*/
return (variable.identifiers.length === 0) ? variable : null;
}
}
/* istanbul ignore next */
return null;
}
/**
* Checkes whether or not a given node is a callback.
* @param {ASTNode} node A node to check.
* @returns {Object}
* {boolean} retv.isCallback - `true` if the node is a callback.
* {boolean} retv.isLexicalThis - `true` if the node is with `.bind(this)`.
*/
function getCallbackInfo(node) {
const retv = { isCallback: false, isLexicalThis: false };
let currentNode = node;
let parent = node.parent;
while (currentNode) {
switch (parent.type) {
// Checks parents recursively.
case "LogicalExpression":
case "ConditionalExpression":
break;
// Checks whether the parent node is `.bind(this)` call.
case "MemberExpression":
if (parent.object === currentNode &&
!parent.property.computed &&
parent.property.type === "Identifier" &&
parent.property.name === "bind" &&
parent.parent.type === "CallExpression" &&
parent.parent.callee === parent
) {
retv.isLexicalThis = (
parent.parent.arguments.length === 1 &&
parent.parent.arguments[0].type === "ThisExpression"
);
parent = parent.parent;
} else {
return retv;
}
break;
// Checks whether the node is a callback.
case "CallExpression":
case "NewExpression":
if (parent.callee !== currentNode) {
retv.isCallback = true;
}
return retv;
default:
return retv;
}
currentNode = parent;
parent = parent.parent;
}
/* istanbul ignore next */
throw new Error("unreachable");
}
/**
* 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",
default: false
},
allowUnboundThis: {
type: "boolean",
default: true
}
},
additionalProperties: false
}
],
fixable: "code"
},
create(context) {
const options = context.options[0] || {};
const allowUnboundThis = options.allowUnboundThis !== false; // default to true
const allowNamedFunctions = options.allowNamedFunctions;
const sourceCode = context.getSourceCode();
/*
* {Array<{this: boolean, super: boolean, meta: boolean}>}
* - this - A flag which shows there are one or more ThisExpression.
* - super - A flag which shows there are one or more Super.
* - meta - A flag which shows there are one or more MethProperty.
*/
let stack = [];
/**
* Pushes new function scope with all `false` flags.
* @returns {void}
*/
function enterScope() {
stack.push({ this: false, super: false, meta: false });
}
/**
* Pops a function scope from the stack.
* @returns {{this: boolean, super: 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;
}
},
Super() {
const info = stack[stack.length - 1];
if (info) {
info.super = true;
}
},
MetaProperty(node) {
const info = stack[stack.length - 1];
if (info && checkMetaProperty(node, "new", "target")) {
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);
if (callbackInfo.isCallback &&
(!allowUnboundThis || !scopeInfo.this || callbackInfo.isLexicalThis) &&
!scopeInfo.super &&
!scopeInfo.meta
) {
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);
}
});
}
}
};
}
};