eslint/lib/rules/no-useless-call.js

   1 /**
   2  * @fileoverview A rule to disallow unnecessary `.call()` and `.apply()`.
   3  * @author Toru Nagashima
   4  */
   5
   6 "use strict";
   7
   8 const astUtils = require("./utils/ast-utils");
   9
  10 //------------------------------------------------------------------------------
  11 // Helpers
  12 //------------------------------------------------------------------------------
  13
  14 /**
  15  * Checks whether or not a node is a `.call()`/`.apply()`.
  16  * @param {ASTNode} node A CallExpression node to check.
  17  * @returns {boolean} Whether or not the node is a `.call()`/`.apply()`.
  18  */
  19 function isCallOrNonVariadicApply(node) {
  20     return (
  21         node.callee.type === "MemberExpression" &&
  22         node.callee.property.type === "Identifier" &&
  23         node.callee.computed === false &&
  24         (
  25             (node.callee.property.name === "call" && node.arguments.length >= 1) ||
  26             (node.callee.property.name === "apply" && node.arguments.length === 2 && node.arguments[1].type === "ArrayExpression")
  27         )
  28     );
  29 }
  30
  31
  32 /**
  33  * Checks whether or not `thisArg` is not changed by `.call()`/`.apply()`.
  34  * @param {ASTNode|null} expectedThis The node that is the owner of the applied function.
  35  * @param {ASTNode} thisArg The node that is given to the first argument of the `.call()`/`.apply()`.
  36  * @param {SourceCode} sourceCode The ESLint source code object.
  37  * @returns {boolean} Whether or not `thisArg` is not changed by `.call()`/`.apply()`.
  38  */
  39 function isValidThisArg(expectedThis, thisArg, sourceCode) {
  40     if (!expectedThis) {
  41         return astUtils.isNullOrUndefined(thisArg);
  42     }
  43     return astUtils.equalTokens(expectedThis, thisArg, sourceCode);
  44 }
  45
  46 //------------------------------------------------------------------------------
  47 // Rule Definition
  48 //------------------------------------------------------------------------------
  49
  50 module.exports = {
  51     meta: {
  52         type: "suggestion",
  53
  54         docs: {
  55             description: "disallow unnecessary calls to `.call()` and `.apply()`",
  56             category: "Best Practices",
  57             recommended: false,
  58             url: "https://eslint.org/docs/rules/no-useless-call"
  59         },
  60
  61         schema: [],
  62
  63         messages: {
  64             unnecessaryCall: "Unnecessary '.{{name}}()'."
  65         }
  66     },
  67
  68     create(context) {
  69         const sourceCode = context.getSourceCode();
  70
  71         return {
  72             CallExpression(node) {
  73                 if (!isCallOrNonVariadicApply(node)) {
  74                     return;
  75                 }
  76
  77                 const applied = node.callee.object;
  78                 const expectedThis = (applied.type === "MemberExpression") ? applied.object : null;
  79                 const thisArg = node.arguments[0];
  80
  81                 if (isValidThisArg(expectedThis, thisArg, sourceCode)) {
  82                     context.report({ node, messageId: "unnecessaryCall", data: { name: node.callee.property.name } });
  83                 }
  84             }
  85         };
  86     }
  87 };