How to Fix Generator Error in Next.js

If you've encountered a Generator Error while working with Next.js, you're not alone. This is one of the most common issues developers face.

What Causes This Error

Generator errors in Next.js happen when using function* or async function* incorrectly — missing the asterisk, forgetting yield, or trying to consume generator objects where arrays are expected.

The Fix

The key is to wrap async generators in a ReadableStream for Next.js streaming route handlers:

// app/api/stream/route.ts
export async function GET() {
  const stream = new ReadableStream({
    async start(controller) {
      for await (const chunk of generateData()) {
        controller.enqueue(new TextEncoder().encode(JSON.stringify(chunk) + "\n"));
      }
      controller.close();
    },
  });
  return new Response(stream, {
    headers: { "Content-Type": "application/x-ndjson" },
  });
}

async function* generateData() {
  for (let i = 0; i < 10; i++) {
    await new Promise(r => setTimeout(r, 100));
    yield { id: i, timestamp: Date.now() };
  }
}

Common Pitfall

A common mistake is to ignore this error during development because it only surfaces under specific conditions. Always test with production-like settings to catch these issues early. If you're working in a team, document this fix in your project's troubleshooting guide so others don't hit the same wall.

Verify the Fix

After applying the fix, restart your Next.js application and verify the error no longer appears in the console or logs. Test both the happy path and edge cases to be thorough. If the error persists, double-check that your changes were saved and the application fully restarted.

Prevention

Want to catch errors like this before they reach production? Bugsly provides real-time error tracking for Next.js applications.