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Viewing file:     no-useless-concat.js (2.9 KB)      -rw-r--r--

/**
 * @fileoverview disallow unnecessary concatenation of template strings
 * @author Henry Zhu
 */
"use strict";

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------

const astUtils = require("./utils/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
//------------------------------------------------------------------------------

/** @type {import('../types').Rule.RuleModule} */
module.exports = {
	meta: {
		type: "suggestion",

		docs: {
			description:
				"Disallow unnecessary concatenation of literals or template literals",
			recommended: false,
			frozen: true,
			url: "https://eslint.org/docs/latest/rules/no-useless-concat",
		},

		schema: [],

		messages: {
			unexpectedConcat: "Unexpected string concatenation of literals.",
		},
	},

	create(context) {
		const sourceCode = context.sourceCode;

		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,
						messageId: "unexpectedConcat",
					});
				}
			},
		};
	},
};