node/tools/eslint/lib/rules/prefer-template.js

/**
 * @fileoverview A rule to suggest using template literals instead of string concatenation.
 * @author Toru Nagashima
 */

"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 === "+";
}

/**
 * Gets the top binary expression node for concatenation in parents of a given node.
 * @param {ASTNode} node - A node to get.
 * @returns {ASTNode} the top binary expression node in parents of a given node.
 */
function getTopConcatBinaryExpression(node) {
    while (isConcatenation(node.parent)) {
        node = node.parent;
    }
    return node;
}

/**
 * Checks whether or not a given binary expression has non string literals.
 * @param {ASTNode} node - A node to check.
 * @returns {boolean} `true` if the node has non string literals.
 */
function hasNonStringLiteral(node) {
    if (isConcatenation(node)) {

        // `left` is deeper than `right` normally.
        return hasNonStringLiteral(node.right) || hasNonStringLiteral(node.left);
    }
    return !astUtils.isStringLiteral(node);
}

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

module.exports = {
    meta: {
        docs: {
            description: "require template literals instead of string concatenation",
            category: "ECMAScript 6",
            recommended: false
        },

        schema: []
    },

    create: function(context) {
        var done = Object.create(null);

        /**
         * Reports if a given node is string concatenation with non string literals.
         *
         * @param {ASTNode} node - A node to check.
         * @returns {void}
         */
        function checkForStringConcat(node) {
            if (!astUtils.isStringLiteral(node) || !isConcatenation(node.parent)) {
                return;
            }

            var topBinaryExpr = getTopConcatBinaryExpression(node.parent);

            // Checks whether or not this node had been checked already.
            if (done[topBinaryExpr.range[0]]) {
                return;
            }
            done[topBinaryExpr.range[0]] = true;

            if (hasNonStringLiteral(topBinaryExpr)) {
                context.report(
                    topBinaryExpr,
                    "Unexpected string concatenation.");
            }
        }

        return {
            Program: function() {
                done = Object.create(null);
            },

            Literal: checkForStringConcat,
            TemplateLiteral: checkForStringConcat
        };
    }
};