Merge pull request #1679 from raunak-rpm/fix/async-generator-headers-1677

fix: preserve headers when throwing from AsyncGenerator

src/compose.ts

@@ -374,7 +374,14 @@ export const isAsync = (v: Function | HookContainer) => {
 	if (isObject && v.isAsync !== undefined) return v.isAsync
 
 	const fn = isObject ? v.fn : v
-	if (fn.constructor.name === 'AsyncFunction') return true
+	// Check for both AsyncFunction and AsyncGeneratorFunction
+	// AsyncGeneratorFunction needs to be treated as async because generator.next()
+	// returns a Promise that may reject (e.g., when throwing from the generator)
+	if (
+		fn.constructor.name === 'AsyncFunction' ||
+		fn.constructor.name === 'AsyncGeneratorFunction'
+	)
+		return true
 
 	const literal: string = fn.toString()
 
@@ -862,7 +869,12 @@ export const composeHandler = ({
 
 	const mapResponse = (r = 'r') => {
 		const after = afterResponse()
-		const response = `${hasSet ? 'mapResponse' : 'mapCompactResponse'}(${saveResponse}${r}${hasSet ? ',c.set' : ''}${mapResponseContext})\n`
+		// When maybeStream is true, mapResponse may return a Promise (from handleStream)
+		// that can reject if the generator throws. We need to await it so the try-catch
+		// can properly catch the rejection and route it to error handling.
+		// Only add await if the function is async (maybeAsync), otherwise it would be a syntax error.
+		const awaitStream = maybeStream && maybeAsync ? 'await ' : ''
+		const response = `${awaitStream}${hasSet ? 'mapResponse' : 'mapCompactResponse'}(${saveResponse}${r}${hasSet ? ',c.set' : ''}${mapResponseContext})\n`
 
 		if (!after) return `return ${response}`
 

test/response/stream.test.ts

@@ -561,6 +561,49 @@ describe('Stream', () => {
 		expect(response.headers.get('content-type')).toBe('text/event-stream')
 	})
 
+	// Issue #1677: Throwing from AsyncGenerator should preserve headers
+	it('should preserve headers when throwing from async generator', async () => {
+		const { status: statusFn } = await import('../../src')
+
+		const app = new Elysia().get('/', async function* ({ set }) {
+			set.headers['access-control-allow-origin'] = '*'
+			set.headers['x-custom-header'] = 'test-value'
+			// Throw before yielding - this is the bug scenario from #1677
+			if (true) throw statusFn(500)
+			yield 'unreachable'
+		})
+
+		const response = await app.handle(req('/'))
+
+		expect(response.status).toBe(500)
+		expect(response.headers.get('access-control-allow-origin')).toBe('*')
+		expect(response.headers.get('x-custom-header')).toBe('test-value')
+	})
+
+	// Issue #1677: onError hook should be called when throwing from generator
+	it('should call onError hook when throwing from async generator', async () => {
+		const { status: statusFn } = await import('../../src')
+		let onErrorCalled = false
+		let errorCode: string | number | undefined
+
+		const app = new Elysia()
+			.onError(({ code }) => {
+				onErrorCalled = true
+				errorCode = code
+			})
+			.get('/', async function* ({ set }) {
+				set.headers['x-custom-header'] = 'test-value'
+				// Throw before yielding - this is the bug scenario from #1677
+				if (true) throw statusFn(500)
+				yield 'unreachable'
+			})
+
+		const response = await app.handle(req('/'))
+
+		expect(response.status).toBe(500)
+		expect(onErrorCalled).toBe(true)
+		expect(errorCode).toBe(500)
+		expect(response.headers.get('x-custom-header')).toBe('test-value')
 	it('handle sse with plugin global hooks and trace', async () => {
 		const PluginA = () =>
 			new Elysia({ name: 'PluginA' })