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/**
* @fileoverview disallow unncessary concatenation of template strings
* @author Henry Zhu
*/
"use strict";
//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------
const astUtils = require("../util/ast-utils");
//------------------------------------------------------------------------------
// Helpers
//------------------------------------------------------------------------------
/**
* Checks whether or not a given node is a concatenation.
* @param {ASTNode} node - A node to check.
* @returns {boolean} `true` if the node is a concatenation.
*/
function isConcatenation(node) {
return node.type === "BinaryExpression" && node.operator === "+";
}
/**
* Checks if the given token is a `+` token or not.
* @param {Token} token - The token to check.
* @returns {boolean} `true` if the token is a `+` token.
*/
function isConcatOperatorToken(token) {
return token.value === "+" && token.type === "Punctuator";
}
/**
* Get's the right most node on the left side of a BinaryExpression with + operator.
* @param {ASTNode} node - A BinaryExpression node to check.
* @returns {ASTNode} node
*/
function getLeft(node) {
let left = node.left;
while (isConcatenation(left)) {
left = left.right;
}
return left;
}
/**
* Get's the left most node on the right side of a BinaryExpression with + operator.
* @param {ASTNode} node - A BinaryExpression node to check.
* @returns {ASTNode} node
*/
function getRight(node) {
let right = node.right;
while (isConcatenation(right)) {
right = right.left;
}
return right;
}
//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------
module.exports = {
meta: {
type: "suggestion",
docs: {
description: "disallow unnecessary concatenation of literals or template literals",
category: "Best Practices",
recommended: false,
url: "https://eslint.org/docs/rules/no-useless-concat"
},
schema: []
},
create(context) {
const sourceCode = context.getSourceCode();
return {
BinaryExpression(node) {
// check if not concatenation
if (node.operator !== "+") {
return;
}
// account for the `foo + "a" + "b"` case
const left = getLeft(node);
const right = getRight(node);
if (astUtils.isStringLiteral(left) &&
astUtils.isStringLiteral(right) &&
astUtils.isTokenOnSameLine(left, right)
) {
const operatorToken = sourceCode.getFirstTokenBetween(left, right, isConcatOperatorToken);
context.report({
node,
loc: operatorToken.loc.start,
message: "Unexpected string concatenation of literals."
});
}
}
};
}
};