--- /dev/null
+/**
+ * @fileoverview A rule to suggest using template literals instead of string concatenation.
+ * @author Toru Nagashima
+ */
+
+"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 === "+";
+}
+
+/**
+ * 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) {
+ let currentNode = node;
+
+ while (isConcatenation(currentNode.parent)) {
+ currentNode = currentNode.parent;
+ }
+ return currentNode;
+}
+
+/**
+ * Checks whether or not a node contains a string literal with an octal or non-octal decimal escape sequence
+ * @param {ASTNode} node A node to check
+ * @returns {boolean} `true` if at least one string literal within the node contains
+ * an octal or non-octal decimal escape sequence
+ */
+function hasOctalOrNonOctalDecimalEscapeSequence(node) {
+ if (isConcatenation(node)) {
+ return (
+ hasOctalOrNonOctalDecimalEscapeSequence(node.left) ||
+ hasOctalOrNonOctalDecimalEscapeSequence(node.right)
+ );
+ }
+
+ // No need to check TemplateLiterals – would throw parsing error
+ if (node.type === "Literal" && typeof node.value === "string") {
+ return astUtils.hasOctalOrNonOctalDecimalEscapeSequence(node.raw);
+ }
+
+ return false;
+}
+
+/**
+ * Checks whether or not a given binary expression has string literals.
+ * @param {ASTNode} node A node to check.
+ * @returns {boolean} `true` if the node has string literals.
+ */
+function hasStringLiteral(node) {
+ if (isConcatenation(node)) {
+
+ // `left` is deeper than `right` normally.
+ return hasStringLiteral(node.right) || hasStringLiteral(node.left);
+ }
+ return astUtils.isStringLiteral(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);
+}
+
+/**
+ * Determines whether a given node will start with a template curly expression (`${}`) when being converted to a template literal.
+ * @param {ASTNode} node The node that will be fixed to a template literal
+ * @returns {boolean} `true` if the node will start with a template curly.
+ */
+function startsWithTemplateCurly(node) {
+ if (node.type === "BinaryExpression") {
+ return startsWithTemplateCurly(node.left);
+ }
+ if (node.type === "TemplateLiteral") {
+ return node.expressions.length && node.quasis.length && node.quasis[0].range[0] === node.quasis[0].range[1];
+ }
+ return node.type !== "Literal" || typeof node.value !== "string";
+}
+
+/**
+ * Determines whether a given node end with a template curly expression (`${}`) when being converted to a template literal.
+ * @param {ASTNode} node The node that will be fixed to a template literal
+ * @returns {boolean} `true` if the node will end with a template curly.
+ */
+function endsWithTemplateCurly(node) {
+ if (node.type === "BinaryExpression") {
+ return startsWithTemplateCurly(node.right);
+ }
+ if (node.type === "TemplateLiteral") {
+ return node.expressions.length && node.quasis.length && node.quasis[node.quasis.length - 1].range[0] === node.quasis[node.quasis.length - 1].range[1];
+ }
+ return node.type !== "Literal" || typeof node.value !== "string";
+}
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+ meta: {
+ type: "suggestion",
+
+ docs: {
+ description: "require template literals instead of string concatenation",
+ category: "ECMAScript 6",
+ recommended: false,
+ url: "https://eslint.org/docs/rules/prefer-template"
+ },
+
+ schema: [],
+ fixable: "code",
+
+ messages: {
+ unexpectedStringConcatenation: "Unexpected string concatenation."
+ }
+ },
+
+ create(context) {
+ const sourceCode = context.getSourceCode();
+ let done = Object.create(null);
+
+ /**
+ * Gets the non-token text between two nodes, ignoring any other tokens that appear between the two tokens.
+ * @param {ASTNode} node1 The first node
+ * @param {ASTNode} node2 The second node
+ * @returns {string} The text between the nodes, excluding other tokens
+ */
+ function getTextBetween(node1, node2) {
+ const allTokens = [node1].concat(sourceCode.getTokensBetween(node1, node2)).concat(node2);
+ const sourceText = sourceCode.getText();
+
+ return allTokens.slice(0, -1).reduce((accumulator, token, index) => accumulator + sourceText.slice(token.range[1], allTokens[index + 1].range[0]), "");
+ }
+
+ /**
+ * Returns a template literal form of the given node.
+ * @param {ASTNode} currentNode A node that should be converted to a template literal
+ * @param {string} textBeforeNode Text that should appear before the node
+ * @param {string} textAfterNode Text that should appear after the node
+ * @returns {string} A string form of this node, represented as a template literal
+ */
+ function getTemplateLiteral(currentNode, textBeforeNode, textAfterNode) {
+ if (currentNode.type === "Literal" && typeof currentNode.value === "string") {
+
+ /*
+ * If the current node is a string literal, escape any instances of ${ or ` to prevent them from being interpreted
+ * as a template placeholder. However, if the code already contains a backslash before the ${ or `
+ * for some reason, don't add another backslash, because that would change the meaning of the code (it would cause
+ * an actual backslash character to appear before the dollar sign).
+ */
+ return `\`${currentNode.raw.slice(1, -1).replace(/\\*(\$\{|`)/gu, matched => {
+ if (matched.lastIndexOf("\\") % 2) {
+ return `\\${matched}`;
+ }
+ return matched;
+
+ // Unescape any quotes that appear in the original Literal that no longer need to be escaped.
+ }).replace(new RegExp(`\\\\${currentNode.raw[0]}`, "gu"), currentNode.raw[0])}\``;
+ }
+
+ if (currentNode.type === "TemplateLiteral") {
+ return sourceCode.getText(currentNode);
+ }
+
+ if (isConcatenation(currentNode) && hasStringLiteral(currentNode) && hasNonStringLiteral(currentNode)) {
+ const plusSign = sourceCode.getFirstTokenBetween(currentNode.left, currentNode.right, token => token.value === "+");
+ const textBeforePlus = getTextBetween(currentNode.left, plusSign);
+ const textAfterPlus = getTextBetween(plusSign, currentNode.right);
+ const leftEndsWithCurly = endsWithTemplateCurly(currentNode.left);
+ const rightStartsWithCurly = startsWithTemplateCurly(currentNode.right);
+
+ if (leftEndsWithCurly) {
+
+ // If the left side of the expression ends with a template curly, add the extra text to the end of the curly bracket.
+ // `foo${bar}` /* comment */ + 'baz' --> `foo${bar /* comment */ }${baz}`
+ return getTemplateLiteral(currentNode.left, textBeforeNode, textBeforePlus + textAfterPlus).slice(0, -1) +
+ getTemplateLiteral(currentNode.right, null, textAfterNode).slice(1);
+ }
+ if (rightStartsWithCurly) {
+
+ // Otherwise, if the right side of the expression starts with a template curly, add the text there.
+ // 'foo' /* comment */ + `${bar}baz` --> `foo${ /* comment */ bar}baz`
+ return getTemplateLiteral(currentNode.left, textBeforeNode, null).slice(0, -1) +
+ getTemplateLiteral(currentNode.right, textBeforePlus + textAfterPlus, textAfterNode).slice(1);
+ }
+
+ /*
+ * Otherwise, these nodes should not be combined into a template curly, since there is nowhere to put
+ * the text between them.
+ */
+ return `${getTemplateLiteral(currentNode.left, textBeforeNode, null)}${textBeforePlus}+${textAfterPlus}${getTemplateLiteral(currentNode.right, textAfterNode, null)}`;
+ }
+
+ return `\`\${${textBeforeNode || ""}${sourceCode.getText(currentNode)}${textAfterNode || ""}}\``;
+ }
+
+ /**
+ * Returns a fixer object that converts a non-string binary expression to a template literal
+ * @param {SourceCodeFixer} fixer The fixer object
+ * @param {ASTNode} node A node that should be converted to a template literal
+ * @returns {Object} A fix for this binary expression
+ */
+ function fixNonStringBinaryExpression(fixer, node) {
+ const topBinaryExpr = getTopConcatBinaryExpression(node.parent);
+
+ if (hasOctalOrNonOctalDecimalEscapeSequence(topBinaryExpr)) {
+ return null;
+ }
+
+ return fixer.replaceText(topBinaryExpr, getTemplateLiteral(topBinaryExpr, null, 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;
+ }
+
+ const 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({
+ node: topBinaryExpr,
+ messageId: "unexpectedStringConcatenation",
+ fix: fixer => fixNonStringBinaryExpression(fixer, node)
+ });
+ }
+ }
+
+ return {
+ Program() {
+ done = Object.create(null);
+ },
+
+ Literal: checkForStringConcat,
+ TemplateLiteral: checkForStringConcat
+ };
+ }
+};
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