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

   1 /**
   2  * @fileoverview disallow unnecessary concatenation of template strings
   3  * @author Henry Zhu
   4  */
   5 "use strict";
   6
   7 //------------------------------------------------------------------------------
   8 // Requirements
   9 //------------------------------------------------------------------------------
  10
  11 const astUtils = require("./utils/ast-utils");
  12
  13 //------------------------------------------------------------------------------
  14 // Helpers
  15 //------------------------------------------------------------------------------
  16
  17 /**
  18  * Checks whether or not a given node is a concatenation.
  19  * @param {ASTNode} node A node to check.
  20  * @returns {boolean} `true` if the node is a concatenation.
  21  */
  22 function isConcatenation(node) {
  23     return node.type === "BinaryExpression" && node.operator === "+";
  24 }
  25
  26 /**
  27  * Checks if the given token is a `+` token or not.
  28  * @param {Token} token The token to check.
  29  * @returns {boolean} `true` if the token is a `+` token.
  30  */
  31 function isConcatOperatorToken(token) {
  32     return token.value === "+" && token.type === "Punctuator";
  33 }
  34
  35 /**
  36  * Get's the right most node on the left side of a BinaryExpression with + operator.
  37  * @param {ASTNode} node A BinaryExpression node to check.
  38  * @returns {ASTNode} node
  39  */
  40 function getLeft(node) {
  41     let left = node.left;
  42
  43     while (isConcatenation(left)) {
  44         left = left.right;
  45     }
  46     return left;
  47 }
  48
  49 /**
  50  * Get's the left most node on the right side of a BinaryExpression with + operator.
  51  * @param {ASTNode} node A BinaryExpression node to check.
  52  * @returns {ASTNode} node
  53  */
  54 function getRight(node) {
  55     let right = node.right;
  56
  57     while (isConcatenation(right)) {
  58         right = right.left;
  59     }
  60     return right;
  61 }
  62
  63 //------------------------------------------------------------------------------
  64 // Rule Definition
  65 //------------------------------------------------------------------------------
  66
  67 /** @type {import('../shared/types').Rule} */
  68 module.exports = {
  69     meta: {
  70         type: "suggestion",
  71
  72         docs: {
  73             description: "Disallow unnecessary concatenation of literals or template literals",
  74             recommended: false,
  75             url: "https://eslint.org/docs/rules/no-useless-concat"
  76         },
  77
  78         schema: [],
  79
  80         messages: {
  81             unexpectedConcat: "Unexpected string concatenation of literals."
  82         }
  83     },
  84
  85     create(context) {
  86         const sourceCode = context.getSourceCode();
  87
  88         return {
  89             BinaryExpression(node) {
  90
  91                 // check if not concatenation
  92                 if (node.operator !== "+") {
  93                     return;
  94                 }
  95
  96                 // account for the `foo + "a" + "b"` case
  97                 const left = getLeft(node);
  98                 const right = getRight(node);
  99
 100                 if (astUtils.isStringLiteral(left) &&
 101                     astUtils.isStringLiteral(right) &&
 102                     astUtils.isTokenOnSameLine(left, right)
 103                 ) {
 104                     const operatorToken = sourceCode.getFirstTokenBetween(left, right, isConcatOperatorToken);
 105
 106                     context.report({
 107                         node,
 108                         loc: operatorToken.loc,
 109                         messageId: "unexpectedConcat"
 110                     });
 111                 }
 112             }
 113         };
 114     }
 115 };