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