2 * @fileoverview disallow unnecessary concatenation of template strings
7 //------------------------------------------------------------------------------
9 //------------------------------------------------------------------------------
11 const astUtils = require("./utils/ast-utils");
13 //------------------------------------------------------------------------------
15 //------------------------------------------------------------------------------
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.
22 function isConcatenation(node) {
23 return node.type === "BinaryExpression" && node.operator === "+";
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.
31 function isConcatOperatorToken(token) {
32 return token.value === "+" && token.type === "Punctuator";
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
40 function getLeft(node) {
43 while (isConcatenation(left)) {
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
54 function getRight(node) {
55 let right = node.right;
57 while (isConcatenation(right)) {
63 //------------------------------------------------------------------------------
65 //------------------------------------------------------------------------------
72 description: "disallow unnecessary concatenation of literals or template literals",
73 category: "Best Practices",
75 url: "https://eslint.org/docs/rules/no-useless-concat"
81 unexpectedConcat: "Unexpected string concatenation of literals."
86 const sourceCode = context.getSourceCode();
89 BinaryExpression(node) {
91 // check if not concatenation
92 if (node.operator !== "+") {
96 // account for the `foo + "a" + "b"` case
97 const left = getLeft(node);
98 const right = getRight(node);
100 if (astUtils.isStringLiteral(left) &&
101 astUtils.isStringLiteral(right) &&
102 astUtils.isTokenOnSameLine(left, right)
104 const operatorToken = sourceCode.getFirstTokenBetween(left, right, isConcatOperatorToken);
108 loc: operatorToken.loc,
109 messageId: "unexpectedConcat"