@wmr-plugins/nomodule

packages/@wmr-plugins_nomodule/README.md

@@ -0,0 +1,6 @@
+# `@wmr-plugins/nomodule`
+
+WMR outputs modern JavaScript bundles by default.
+This plugin creates legacy versions of your bundles using [@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env) and serves those versions to older browsers.
+
+New browsers get the new stuff, old browsers get the old stuff.

packages/@wmr-plugins_nomodule/index.js

@@ -0,0 +1,91 @@
+import babel from '@babel/standalone';
+
+/** @param {import('../../types').Options} options */
+export default function (options) {
+	if (options.mode && options.mode !== 'build') return;
+	options.plugins.push(nomodulePlugin({}));
+}
+
+/**
+ * @param {object} [options]
+ * @returns {import('rollup').Plugin}
+ */
+function nomodulePlugin({} = {}) {
+	return {
+		name: '@wmr-plugins/nomodule',
+		async generateBundle(opts, bundle) {
+			const downleveled = new Map();
+			for (const fileName in bundle) {
+				const chunk = bundle[fileName];
+				if (chunk.type !== 'chunk') continue;
+				const legacy = downlevel(chunk.code, fileName);
+				if (!legacy) continue;
+				const legacyFileName = chunk.fileName.replace(/\.js/, '.legacy.js');
+				this.emitFile({
+					type: 'asset',
+					fileName: legacyFileName,
+					source: legacy
+				});
+				downleveled.set(fileName, legacyFileName);
+			}
+
+			// Update all the HTML files with <script type=module> to add legacy loading via Shimport+Polyfills
+			for (const fileName in bundle) {
+				const asset = bundle[fileName];
+				if (asset.type !== 'asset' || typeof asset.source !== 'string' || !asset.fileName.match(/\.html$/)) continue;
+				if (!/<script(?:\s[^>]*)?\s+type=(['"])module\1/.test(asset.source)) continue;
+				// this is gross obviously
+				const POLYFILL = 'https://unpkg.com/@babel/polyfill@7.12.1/browser.js'; // https://unpkg.com/regenerator-runtime
+				const SHIMPORT = 'https://unpkg.com/shimport@2.0.4/index.js';
+				const script = `<script nomodule src="${POLYFILL}"></script><script nomodule src="${SHIMPORT}"></script><script nomodule>[].forEach.call(document.querySelectorAll('script[type=module]'),function(n){__shimport__.load(n.src.replace(/\\.js$/,'.legacy.js'))})</script>`;
+				if (/<\/body>/.test(asset.source)) {
+					asset.source = asset.source.replace(/<\/body>/, `${script}</body>`);
+				} else {
+					asset.source += script;
+				}
+			}
+		}
+	};
+}
+
+function downlevel(code, fileName) {
+	const result = babel.transform(code, {
+		compact: true,
+		minified: true,
+		configFile: false,
+		babelrc: false,
+		filename: fileName,
+		presets: [
+			[
+				'env',
+				{
+					targets: 'defaults',
+					shippedProposals: true,
+					loose: true,
+					bugfixes: true,
+					corejs: false,
+					useBuiltIns: false,
+					modules: false
+				}
+			]
+		],
+		plugins: [
+			{
+				visitor: {
+					ImportDeclaration(path) {
+						path.node.source.value = path.node.source.value.replace(/(\.legacy)?\.js$/, '.legacy.js');
+					},
+					Import(path) {
+						const p = path.parentPath;
+						if (!p.isCallExpression()) return;
+						const arg = p.get('arguments.0');
+						// the things I do to make TypeScript happy...
+						if (Array.isArray(arg) || !arg.isStringLiteral()) return;
+						arg.node.value = arg.node.value.replace(/(\.legacy)?\.js$/, '.legacy.js');
+					}
+				}
+			}
+		]
+	});
+	return result.code;
+}

packages/@wmr-plugins_nomodule/package.json

@@ -0,0 +1,27 @@
+{
+	"name": "@wmr-plugins/nomodule",
+	"publishConfig": {
+		"access": "public"
+	},
+	"version": "0.1.0",
+	"description": "Generate legacy bundles and serve them to older browsers as a fallback.",
+	"main": "./dist/index.js",
+	"type": "module",
+	"exports": {
+		"import": "./index.js",
+		"require": "./dist/index.cjs"
+	},
+	"scripts": {
+		"prepare": "npx cjyes"
+	},
+	"author": "The Preact Authors (https://preactjs.com)",
+	"repository": "preactjs/wmr",
+	"license": "MIT",
+	"files": [
+		"index.js",
+		"dist"
+	],
+	"dependencies": {
+		"@babel/standalone": "^7.11.6"
+	}
+}

src/lib/normalize-options.js

@@ -56,19 +56,21 @@ export async function normalizeOptions(options, mode) {
 	const hasMjsConfig = await isFile(resolve(options.root, 'wmr.config.mjs'));
 	if (hasMjsConfig || (await isFile(resolve(options.root, 'wmr.config.js')))) {
 		let custom,
-			initialConfigFile = hasMjsConfig ? 'wmr.config.mjs' : 'wmr.config.js';
+			initialConfigFile = hasMjsConfig ? 'wmr.config.mjs' : 'wmr.config.js',
+			initialError;
 		try {
 			const resolved = resolve(options.root, initialConfigFile);
 			// Note: the eval() below is to prevent Rollup from transforming import() and require().
 			// Using the native functions allows us to load ESM and CJS with Node's own semantics.
 			try {
 				custom = await eval('(x => import(x))')(resolved);
 			} catch (err) {
+				initialError = err;
 				custom = eval('(x => require(x))')(resolved);
 			}
 		} catch (e) {
 			if (hasMjsConfig || !/import statement/.test(e)) {
-				throw Error(`Failed to load ${initialConfigFile}\n${e}`);
+				throw Error(`Failed to load ${initialConfigFile}\n${initialError}\n${e}`);
 			}
 		}
 		Object.defineProperty(options, '_config', { value: custom });