/**
* @fileoverview A rule to suggest using template literals instead of string concatenation.
* @author Toru Nagashima
*/
"use strict";
//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------
const 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 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: {
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"
},
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(/\\*(\${|`)/g, 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]}`, "g"), 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 || ""}}\``;
}
/**
* 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,
message: "Unexpected string concatenation.",
fix(fixer) {
return fixer.replaceText(topBinaryExpr, getTemplateLiteral(topBinaryExpr, null, null));
}
});
}
}
return {
Program() {
done = Object.create(null);
},
Literal: checkForStringConcat,
TemplateLiteral: checkForStringConcat
};
}
};