AWS Lambda plans to remove support for callback-based function handlers

[codygordon]

Motivation

Ref: aws/aws-lambda-nodejs-runtime-interface-client#137

Callback handler syntax is being deprecated for the new Node 24 Lambda runtime and should be replaced with the async syntax.

I didn't see any open issues or PRs related to this, apologies if this is duplicative or if there is already an option to change this behavior under the hood. Otherwise, if nobody else is working on this I can give it a shot.

Example

exports.handler = (event, context, callback) => {
  // ... handler logic
  callback(null, { statusCode: 200, body: 'Success!' });
};

to:

export const handler = async (event, context) => {
  // ... handler logic
  return { statusCode: 200, body: 'Success!' };
};

[mcollina]

A PR would be highly welcomed. How can we detect/decide which one we should use?

[codygordon]

It seems the plan is just to remove callback in the soon-to-be-released Node 24 runtime (updated the issue description with this detail), so I believe env.AWS_EXECUTION_ENV === 'nodejs24.x' would do the trick. It's possible the async syntax works in any current Node runtime env, but I'm not entirely sure about that so this conditional is probably the safest bet for backward compatability given that older runtimes will still allow it.

Edit: actually, for forward-compatability that strict check on 24 won't work, it would have to be a check on the list of runtimes before 24 to use callback, i.e. nodejs18.x, nodejs20.x, nodejs22.x.

[adrai]

Lets wait for the official decision of AWS... currently it's not officially decided.
As soon as they decide to remove it, we can create a new major version without callback signature...

[adrai]

Try v6.0.0.... this is a preliminary step before definitely removing the callback functionality.