feat: migrate to native ESM with no build step

- Rename src/ to lib/ — source files are shipped directly, no compilation
- Add "type": "module" to package.json for native ESM support (Node 18+)
- Convert bin/lessc, test files, and build scripts from CJS to ESM
- Rename Gruntfile.js and .eslintrc.js to .cjs (must remain CommonJS)
- Add .js extensions to all relative import paths for ESM resolution
- Use createRequire() for optional dependency resolution (npm packages, JSON)
- Configure TypeScript for check-only mode (noEmit: true, allowJs: true)
- Update Rollup config to read from lib/ directly
- Update CI matrix to drop Node 16 (minimum Node 18+)
- Browser build is smaller: 500KB (was 509KB), minified 153KB (was 158KB)
- All 139 tests pass

Author: matthew-dean

.github/workflows/ci.yml

@@ -23,8 +23,6 @@ jobs:
             node: 'lts/-1'
           - os: ubuntu-latest
             node: 'lts/-2'
-          - os: ubuntu-latest
-            node: 'lts/-3'
 
     runs-on: ${{ matrix.os }}
     # This has copy/paste steps and should be refactored using DRY

packages/less/.eslintrc.js packages/less/.eslintrc.cjspackages/less/.eslintrc.js renamed to packages/less/.eslintrc.cjs

@@ -1,6 +1,5 @@
 # project-specific
 tmp
-lib
 test/browser/less.min.js
 test/browser/less.min.js.map
 test/sourcemaps/**/*.map

packages/less/.gitignore

@@ -193,27 +193,16 @@ module.exports = function(grunt) {
                 }
             },
             build: {
-                command: [
-                    /** Browser runtime */
-                    "node build/rollup.js --dist",
-                    /** Node.js runtime */
-                    "npm run build"
-                ].join(" && ")
+                command: "node build/rollup.js --dist"
             },
             testbuild: {
-                command: [
-                    "npm run build",
-                    "node build/rollup.js --browser --out=./tmp/browser/less.min.js"
-                ].join(" && ")
-            },
-            testcjs: {
-                command: "npm run build"
+                command: "node build/rollup.js --browser --out=./tmp/browser/less.min.js"
             },
             testbrowser: {
                 command: "node build/rollup.js --browser --out=./tmp/browser/less.min.js"
             },
             test: {
-                command: 'npx ts-node test/test-es6.ts && node test/index.js'
+                command: 'node test/test-es6.js && node test/index.js'
             },
             generatebrowser: {
                 command: 'node test/browser/generator/generate.js'
@@ -265,11 +254,11 @@ module.exports = function(grunt) {
         eslint: {
             target: [
                 "test/**/*.js",
-                "src/less*/**/*.js",
+                "lib/less*/**/*.js",
                 "!test/less/errors/plugin/plugin-error.js"
             ],
             options: {
-                configFile: ".eslintrc.js",
+                configFile: ".eslintrc.cjs",
                 fix: true
             }
         },
@@ -381,9 +370,8 @@ module.exports = function(grunt) {
     // Run shell plugin test
     grunt.registerTask("shell-plugin", ["shell:plugin"]);
 
-    // Quickly build and run Node tests
+    // Quickly run Node tests (no build step needed)
     grunt.registerTask("quicktest", [
-        "shell:testcjs",
         "shell:test"
     ]);
 
@@ -397,7 +385,6 @@ module.exports = function(grunt) {
 
     // Run benchmark
     grunt.registerTask("benchmark", [
-        "shell:testcjs",
         "shell:benchmark"
     ]);
 };

packages/less/Gruntfile.js packages/less/Gruntfile.cjspackages/less/Gruntfile.js renamed to packages/less/Gruntfile.cjs

@@ -2,18 +2,17 @@
 
 /* eslint indent: [2, 2, {"SwitchCase": 1}] */
 
-'use strict';
+import path from 'path';
+import os from 'os';
+import { createRequire } from 'module';
+import fs from '../lib/less-node/fs.js';
+import * as utils from '../lib/less/utils.js';
+import * as Constants from '../lib/less/constants.js';
+import less from '../lib/less-node/index.js';
 
-var path = require('path');
-var fs = require('../lib/less-node/fs').default;
-var os = require('os');
-var utils = require('../lib/less/utils');
-var Constants = require('../lib/less/constants');
-
-var less = require('../lib/less-node').default;
+const require = createRequire(import.meta.url);
 
 var errno;
-var mkdirp;
 
 try {
   errno = require('errno');
@@ -108,22 +107,22 @@ function render() {
     }
 
     // Handle explicit sourceMapFullFilename (from --source-map=filename)
-    // Normalization of other options (sourceMapBasepath, sourceMapRootpath, etc.) 
+    // Normalization of other options (sourceMapBasepath, sourceMapRootpath, etc.)
     // is handled automatically in parse-tree.js
     if (sourceMapOptions.sourceMapFullFilename && !sourceMapFileInline) {
       var mapFilename = path.resolve(process.cwd(), sourceMapOptions.sourceMapFullFilename);
       var mapDir = path.dirname(mapFilename);
-      
+
       if (output) {
         var outputDir = path.dirname(output);
         // Set sourceMapOutputFilename relative to map directory
         sourceMapOptions.sourceMapOutputFilename = path.join(
-          path.relative(mapDir, outputDir), 
+          path.relative(mapDir, outputDir),
           path.basename(output)
         );
         // Set sourceMapFilename relative to output directory (for sourceMappingURL comment)
         sourceMapOptions.sourceMapFilename = path.join(
-          path.relative(outputDir, mapDir), 
+          path.relative(outputDir, mapDir),
           path.basename(sourceMapOptions.sourceMapFullFilename)
         );
       } else {
@@ -151,6 +150,7 @@ function render() {
     return;
   }
 
+  var mkdirp;
   var ensureDirectory = function ensureDirectory(filepath) {
     var dir = path.dirname(filepath);
     var cmd;
@@ -189,7 +189,7 @@ function render() {
 
       // To fix https://github.com/less/less.js/issues/3646
       output = output.toString();
-      
+
       fs.writeFile(filename, output, 'utf8', function (err) {
         if (err) {
           var description = 'Error: ';
@@ -404,7 +404,7 @@ function processPluginQueue() {
       case 'silent':
         options.silent = silent = true;
         break;
-      
+
       case 'quiet':
         options.quiet = quiet = true;
         break;
@@ -539,7 +539,7 @@ function processPluginQueue() {
         }
 
         break;
-      
+
       case 'ie-compat':
         pendingDeprecations.push('Warning: The --ie-compat option is deprecated, as it has no effect on compilation.');
         break;
@@ -652,7 +652,7 @@ function processPluginQueue() {
       case 'disable-plugin-rule':
         options.disablePluginRule = true;
         break;
-  
+
       default:
         queuePlugins.push({
           name: arg,
@@ -675,4 +675,4 @@ function processPluginQueue() {
   } else {
     render();
   }
-})();
+})();

packages/less/bin/lessc

@@ -1,10 +1,13 @@
+import { createRequire } from 'module';
+
+const require = createRequire(import.meta.url);
 const pkg = require('./../package.json');
 
-module.exports = 
+export default
 `/**
  * Less - ${ pkg.description } v${ pkg.version }
  * http://lesscss.org
- * 
+ *
  * Copyright (c) 2009-${new Date().getFullYear()}, ${ pkg.author.name } <${ pkg.author.email }>
  * Licensed under the ${ pkg.license } License.
  *

packages/less/build/banner.js

@@ -1,21 +1,23 @@
-const rollup = require('rollup');
-const typescript = require('rollup-plugin-typescript2');
-const commonjs = require('@rollup/plugin-commonjs');
-const json = require('@rollup/plugin-json');
-const resolve = require('@rollup/plugin-node-resolve').nodeResolve;
-const terser = require('rollup-plugin-terser').terser;
-const banner = require('./banner');
-const path = require('path');
+import { rollup } from 'rollup';
+import commonjs from '@rollup/plugin-commonjs';
+import json from '@rollup/plugin-json';
+import { nodeResolve as resolve } from '@rollup/plugin-node-resolve';
+import { terser } from 'rollup-plugin-terser';
+import banner from './banner.js';
+import path from 'path';
+import { fileURLToPath } from 'url';
+import minimist from 'minimist';
 
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
 const rootPath = path.join(__dirname, '..');
 
-const args = require('minimist')(process.argv.slice(2));
+const args = minimist(process.argv.slice(2));
 
 let outDir = args.dist ? './dist' : './tmp';
 
 async function buildBrowser() {
-    let bundle = await rollup.rollup({
-        input: './src/less-browser/bootstrap.js',
+    let bundle = await rollup({
+        input: './lib/less-browser/bootstrap.js',
         output: [
             {
                 file: 'less.js',
@@ -30,18 +32,6 @@ async function buildBrowser() {
             resolve(),
             commonjs(),
             json(),
-            typescript({
-                verbosity: 2,
-                tsconfigDefaults: {
-                    compilerOptions: {
-                        allowJs: true,
-                        sourceMap: true,
-                        target: 'ES5'
-                    }
-                },
-                include: [ '*.ts', '**/*.ts', '*.js', '**/*.js' ],
-                exclude: ['node_modules'] // only transpile our source code
-            }),
             terser({
                 compress: true,
                 include: [/^.+\.min\.js$/],
@@ -65,7 +55,7 @@ async function buildBrowser() {
             format: 'umd',
             name: 'less',
             banner
-        }); 
+        });
     }
 
     if (!args.out || args.out.indexOf('less.min.js') > -1) {