Add loader supporting mixed-format packages (#8)

Author: JakobJingleheimer

mixed-format/fixture/cjs.js

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+exports.qux = require('./cjs2.js');

mixed-format/fixture/cjs2.js

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+module.exports = 'zed';

mixed-format/fixture/esm.js

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+export const foo = 'bar';

mixed-format/fixture/package.json

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+{
+	"type": "commonjs"
+}

mixed-format/loader.mjs

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+export async function load(url, context, nextLoad) {
+  if (!url.endsWith('.js')) return nextLoad(url);
+
+  const nextResult = await nextLoad(url, { ...context, format: 'module' })
+    .then((result) => {
+      if (containsCJS(result.source)) { throw new Error('CommonJS'); }
+
+      return result;
+    })
+    .catch(async (err) => {
+      if (
+        (err?.message.includes('require') && err.includes('import'))
+        || err?.message.includes('CommonJS')
+      ) {
+        return { format: 'commonjs' };
+      }
+
+      throw err;
+    });
+
+  return nextResult;
+}
+
+function containsCJS(source) {
+  const src = '' + source;
+
+  // A realistic version of this loader would use a parser like Acorn to check for actual `module.exports` syntax
+  if (src.match(/exports[\.( ?=)]/)) { return true };
+
+  if (
+    src.match(/require\(/)
+    && !src.match(/createRequire\(/)
+  ) return true;
+}

mixed-format/test.mjs

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+import assert from 'node:assert';
+
+import { foo } from './fixture/esm.js';
+import { qux } from './fixture/cjs.js';
+
+
+assert.strictEqual(foo, 'bar');
+assert.strictEqual(qux, 'zed');