--esm flag for emitting es modules

vercel · Jun 26, 2021 · 7164f29 · 7164f29
1 parent a529543
commit 7164f29
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Showing 3 changed files with 30 additions and 12 deletions.
diff --git a/src/cli.js b/src/cli.js
@@ -5,7 +5,7 @@ const glob = require("glob");
 const shebangRegEx = require("./utils/shebang");
 const rimraf = require("rimraf");
 const crypto = require("crypto");
-const { writeFileSync, unlink, existsSync, symlinkSync } = require("fs");
+const { writeFileSync, unlink, existsSync, symlinkSync, mkdir } = require("fs");
 const mkdirp = require("mkdirp");
 const { version: nccVersion } = require('../package.json');
 
@@ -24,6 +24,8 @@ Commands:
 
 Options:
   -o, --out [file]         Output directory for build (defaults to dist)
+  
+  --esm                    Create an ES Module build output
   -m, --minify             Minify output
   -C, --no-cache           Skip build cache population
   -s, --source-map         Generate source map
@@ -129,6 +131,7 @@ async function runCmd (argv, stdout, stderr) {
     args = require("arg")({
       "--debug": Boolean,
       "-d": "--debug",
+      "--esm": Boolean,
       "--external": [String],
       "-e": "--external",
       "--out": String,
@@ -218,6 +221,8 @@ async function runCmd (argv, stdout, stderr) {
       );
       if (existsSync(outDir))
         rimraf.sync(outDir);
+      mkdirp.sync(outDir);
+      writeFileSync(eval('resolve')(outDir, 'package.json'), args['--esm'] ? '{ "type": "module" }' : '{}');
       run = true;
 
     // fallthrough
@@ -228,12 +233,13 @@ async function runCmd (argv, stdout, stderr) {
       let startTime = Date.now();
       let ps;
       const buildFile = eval("require.resolve")(resolve(args._[1] || "."));
-      const ext = buildFile.endsWith('.cjs') ? '.cjs' : '.js';
+      const ext = !args['--esm'] && buildFile.endsWith('.cjs') ? '.cjs' : '.js';
       const ncc = require("./index.js")(
         buildFile,
         {
           debugLog: args["--debug"],
           minify: args["--minify"],
+          esm: args["--esm"],
           externals: args["--external"],
           sourceMap: args["--source-map"] || run,
           sourceMapRegister: args["--no-source-map-register"] ? false : undefined,

diff --git a/src/index.js b/src/index.js
@@ -36,8 +36,9 @@ function ncc (
   {
     cache,
     customEmit = undefined,
+    esm = false,
     externals = [],
-    filename = 'index' + (entry.endsWith('.cjs') ? '.cjs' : '.js'),
+    filename = 'index' + (!esm && entry.endsWith('.cjs') ? '.cjs' : '.js'),
     minify = false,
     sourceMap = false,
     sourceMapRegister = true,
@@ -233,7 +234,8 @@ function ncc (
     },
     amd: false,
     experiments: {
-      topLevelAwait: true
+      topLevelAwait: true,
+      outputModule: true
     },
     optimization: {
       nodeEnv: false,
@@ -258,8 +260,9 @@ function ncc (
       path: "/",
       // Webpack only emits sourcemaps for files ending in .js
       filename: ext === '.cjs' ? filename + '.js' : filename,
-      libraryTarget: "commonjs2",
-      strictModuleExceptionHandling: true
+      libraryTarget: esm ? 'module' : 'commonjs2',
+      strictModuleExceptionHandling: true,
+      module: esm
     },
     resolve: {
       extensions: SUPPORTED_EXTENSIONS,
@@ -465,12 +468,13 @@ function ncc (
       let result;
       try {
         result = await terser.minify(code, {
+          module: esm,
           compress: false,
           mangle: {
             keep_classnames: true,
             keep_fnames: true
           },
-          sourceMap: sourceMap ? {
+          sourceMap: map ? {
             content: map,
             filename,
             url: `${filename}.map`
@@ -483,10 +487,10 @@ function ncc (
 
         ({ code, map } = {
           code: result.code,
-          map: sourceMap ? JSON.parse(result.map) : undefined
+          map: map ? JSON.parse(result.map) : undefined
         });
       }
-      catch {
+      catch (e) {
         console.log('An error occurred while minifying. The result will not be minified.'); 
       }
     }
@@ -511,9 +515,10 @@ function ncc (
         `if (cachedData) process.on('exit', () => { try { writeFileSync(basename + '.cache', script.createCachedData()); } catch(e) {} });\n`;
     }
 
-    if (sourceMap && sourceMapRegister) {
-      code = `require('./sourcemap-register${ext}');` + code;
-      assets[`sourcemap-register${ext}`] = { source: fs.readFileSync(`${__dirname}/sourcemap-register.js.cache.js`), permissions: defaultPermissions };
+    if (map && sourceMapRegister) {
+      const registerExt = esm ? '.cjs' : ext;
+      code = (esm ? `import './sourcemap-register${registerExt}';` : `require('./sourcemap-register${registerExt}');`) + code;
+      assets[`sourcemap-register${registerExt}`] = { source: fs.readFileSync(`${__dirname}/sourcemap-register.js.cache.js`), permissions: defaultPermissions };
     }
 
     if (shebangMatch) {

diff --git a/test/cli.js b/test/cli.js
@@ -103,5 +103,12 @@
     expect (code, stdout) { 
       return code === 0 && stdout.indexOf('ncc built-in') !== -1;
     },
+  },
+  {
+    args: ["build", "-o", "tmp", "test/fixtures/module.cjs", "--esm"],
+    expect (code, stdout) {
+      const fs = require('fs');
+      return code === 0 && fs.readFileSync('tmp/index.js', 'utf8').toString().indexOf('export {') !== -1;
+    }
   }
 ]