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

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

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

var astUtils = require("../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 === "+";
}

/**
 * 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) {
    var 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) {
    var right = node.right;

    while (isConcatenation(right)) {
        right = right.left;
    }
    return right;
}

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------

module.exports = {
    meta: {
        docs: {
            description: "disallow unnecessary concatenation of literals or template literals",
            category: "Best Practices",
            recommended: false
        },

        schema: []
    },

    create: function(context) {
        return {
            BinaryExpression: function(node) {

                // check if not concatenation
                if (node.operator !== "+") {
                    return;
                }

                // account for the `foo + "a" + "b"` case
                var left = getLeft(node);
                var right = getRight(node);

                if (astUtils.isStringLiteral(left) &&
                    astUtils.isStringLiteral(right) &&
                    astUtils.isTokenOnSameLine(left, right)
                ) {

                    // move warning location to operator
                    var operatorToken = context.getTokenAfter(left);

                    while (operatorToken.value !== "+") {
                        operatorToken = context.getTokenAfter(operatorToken);
                    }

                    context.report(
                        node,
                        operatorToken.loc.start,
                        "Unexpected string concatenation of literals.");
                }
            }
        };
    }
};