--- /dev/null
+/**
+ * @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);
+ }
+ });
+ }
+ }
+ };
+ }
+};
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