Disable format conversion when detecting format (#2766)

* Disable format conversion when detecting format, closes #1668

Wrangler automatically detects whether your code is a `modules` or
`service-worker` format Worker based on the presence of a `default`
`export`. This check currently works by building your entrypoint with
`esbuild` and looking at the output metafile.

Previously, we were passing `format: "esm"` to `esbuild` when
performing this check, which enables *format conversion*. This may
introduce `export default` into the built code, even if it wasn't
there to start with, resulting in incorrect format detections.

This change removes `format: "esm"` which disables format conversion
when bundling is disabled: https://esbuild.github.io/api/#format.

We may want to use a package like `es-module-lexer` in the future,
but this issue is affecting users, and this is probably the simplest
fix.

Closes #1668
Closes #2396
Ref #2737

* fixup! Disable format conversion when detecting format, closes #1668

Author: mrbbot

.changeset/nervous-scissors-roll.md

@@ -0,0 +1,11 @@
+---
+"wrangler": patch
+---
+
+fix: correctly detect `service-worker` format when using `typeof module`
+
+Wrangler automatically detects whether your code is a `modules` or `service-worker` format Worker based on the presence of a `default` `export`. This check currently works by building your entrypoint with `esbuild` and looking at the output metafile.
+
+Previously, we were passing `format: "esm"` to `esbuild` when performing this check, which enables _format conversion_. This may introduce `export default` into the built code, even if it wasn't there to start with, resulting in incorrect format detections.
+
+This change removes `format: "esm"` which disables format conversion when bundling is disabled. See https://esbuild.github.io/api/#format for more details.

packages/wrangler/src/__tests__/guess-worker-format.test.ts

@@ -32,6 +32,41 @@ describe("guess worker format", () => {
 		expect(guess).toBe("service-worker");
 	});
 
+	it('should detect a "service-worker" worker using `typeof module`', async () => {
+		await writeFile("./index.ts", "typeof module");
+		const guess = await guessWorkerFormat(
+			path.join(process.cwd(), "./index.ts"),
+			process.cwd(),
+			undefined
+		);
+		expect(guess).toBe("service-worker");
+	});
+
+	it('should detect a "service-worker" worker using imports', async () => {
+		await writeFile(
+			"./dep.ts",
+			`
+			const value = 'thing';
+			export default value;
+			`
+		);
+		await writeFile(
+			"./index.ts",
+			`
+			import value from './dep.ts';
+			addEventListener('fetch', (event) => {
+				event.respondWith(new Response(value));
+			});
+			`
+		);
+		const guess = await guessWorkerFormat(
+			path.join(process.cwd(), "./index.ts"),
+			process.cwd(),
+			undefined
+		);
+		expect(guess).toBe("service-worker");
+	});
+
 	it("should throw an error when the hint doesn't match the guess (modules - service-worker)", async () => {
 		await writeFile("./index.ts", "export default {};");
 		await expect(

packages/wrangler/src/entry.ts

@@ -151,7 +151,6 @@ export default async function guessWorkerFormat(
 		absWorkingDir: entryWorkingDirectory,
 		metafile: true,
 		bundle: false,
-		format: "esm",
 		target: "es2022",
 		write: false,
 		loader: {