How to solve the callback response already delivered warning when writing AWS lambda

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If you have ever receive this warning from the log of your lambda,

WARNING: Callback/response already delivered. Did your function invoke the callback and also return a promise? For more details, see: https://docs.aws.amazon.com/lambda/latest/dg/nodejs-prog-model-handler.html

I got this when implementing Cognito lambda trigger of the PreSignUp step.

Let’s see how to solve it.

1. What causes this

If you use Lambda proxy, and async/await. And you see the above warning, that means you are returning via invoking the callback(). Even though you are not return any promise.

something like:

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const handler = async (event, context, callback) => {
  await myAsyncFunc()

  callback(null, {
    statusCode: 200.
    body: JSON.stringify({message:"ok"})
  })
}

You will be shocked to see you are not returning any Promise, just invoking that callback() to return value. So basically, the error message does not apply at all.

2. Solution for the lambda handler

So, instead of using callback(), you just return the response directly.

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const handler = async (event, context, callback) => {
  await myAsyncFunc()

  return {
    statusCode: 200.
    body: JSON.stringify({message:"ok"})
  };
}

Set the statusCode to 400 when returning an error.

3. Solution for the Cognito trigger lambda

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const cognitoPreSignUpTrigger = async event => {
  await myAsyncFunc();

  return event;
};

You simple just return the event, no worries about the error, simply LET IT THROW, it will be caught by the gateway and being returned.

The trouble comes from the confusion of the error message, and all the online document for writing Cognito trigger is callback() based.

4. End

That’s it, hope it helps.

Thanks for reading!

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