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     const callee = astUtils.skipChainExpression(node.callee);
  21
  22     return (
  23         callee.type === "MemberExpression" &&
  24         callee.property.type === "Identifier" &&
  25         callee.computed === false &&
  26         (
  27             (callee.property.name === "call" && node.arguments.length >= 1) ||
  28             (callee.property.name === "apply" && node.arguments.length === 2 && node.arguments[1].type === "ArrayExpression")
  29         )
  30     );
  31 }
  32
  33
  34 /**
  35  * Checks whether or not `thisArg` is not changed by `.call()`/`.apply()`.
  36  * @param {ASTNode|null} expectedThis The node that is the owner of the applied function.
  37  * @param {ASTNode} thisArg The node that is given to the first argument of the `.call()`/`.apply()`.
  38  * @param {SourceCode} sourceCode The ESLint source code object.
  39  * @returns {boolean} Whether or not `thisArg` is not changed by `.call()`/`.apply()`.
  40  */
  41 function isValidThisArg(expectedThis, thisArg, sourceCode) {
  42     if (!expectedThis) {
  43         return astUtils.isNullOrUndefined(thisArg);
  44     }
  45     return astUtils.equalTokens(expectedThis, thisArg, sourceCode);
  46 }
  47
  48 //------------------------------------------------------------------------------
  49 // Rule Definition
  50 //------------------------------------------------------------------------------
  51
  52 module.exports = {
  53     meta: {
  54         type: "suggestion",
  55
  56         docs: {
  57             description: "disallow unnecessary calls to `.call()` and `.apply()`",
  58             recommended: false,
  59             url: "https://eslint.org/docs/rules/no-useless-call"
  60         },
  61
  62         schema: [],
  63
  64         messages: {
  65             unnecessaryCall: "Unnecessary '.{{name}}()'."
  66         }
  67     },
  68
  69     create(context) {
  70         const sourceCode = context.getSourceCode();
  71
  72         return {
  73             CallExpression(node) {
  74                 if (!isCallOrNonVariadicApply(node)) {
  75                     return;
  76                 }
  77
  78                 const callee = astUtils.skipChainExpression(node.callee);
  79                 const applied = astUtils.skipChainExpression(callee.object);
  80                 const expectedThis = (applied.type === "MemberExpression") ? applied.object : null;
  81                 const thisArg = node.arguments[0];
  82
  83                 if (isValidThisArg(expectedThis, thisArg, sourceCode)) {
  84                     context.report({ node, messageId: "unnecessaryCall", data: { name: callee.property.name } });
  85                 }
  86             }
  87         };
  88     }
  89 };