eslint-plugin-mocha/lib/rules/prefer-arrow-callback.js

'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 astUtils = 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;
}

/**
 * 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, context) {
    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;
}

/**
 * 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: {
        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 options = context.options[0] || {};

        // allowUnboundThis defaults to true
        const allowUnboundThis = options.allowUnboundThis !== false;
        const allowNamedFunctions = options.allowNamedFunctions;
        const sourceCode = context.getSourceCode();

        /*
         * {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);
                        }
                    });
                }
            }
        };
    }
};