AWS Lambda done callback() function in Node

Question

The microservice-http-endpoint starter function in Lambda contains the following code to end the function:

const done = (err, res) => callback(null, {
    statusCode: err ? '400' : '200',
    body: err ? err.message : JSON.stringify(res),
    headers: {
        'Content-Type': 'application/json',
    },
});

After writing a Lambda-only API with a few endpoints I found myself with this same code in all my functions. My first instinct was to put this into a module to make the code more DRY but if I do that the callback() function will become out of scope.

I am only returning JSON from my functions so this would be the only possible Content-Type in my specific use-case.

How are people dealing with this? Is there a practical (and elegant) way to encapsulate this code somehow? Or maybe a better alternative?

Just set global.callback = callback as the first line of your Lambda handler function and then it will never be out of scope. — idbehold
– idbehold, Commented Jan 30, 2017 at 16:44
@idbehold sending this to global feels iffy. Especially something so generic as "callback". — Julian
– Julian, Commented Jan 30, 2017 at 19:19
But Lambda container instances will only run your handler one at a time. If you invoke a Lambda function and then try to invoke the same Lambda function a second time before the first finishes, the second invocation will get a brand new container to run in. Only once your handler function has finished (or timed out) will that container be available again. So the global really shouldn't matter in the context of a Lambda function. You could always change the global name to something more specific if it makes you feel better global.MY_SUPER_SECRET_CALLBACK = callback. — idbehold
– idbehold, Commented Jan 30, 2017 at 19:28

bebbi · Accepted Answer · 2017-03-03 12:04:18Z

1

Just nest the functions:

const regularize = (awsCallback) => (err, res) => {
  awsCallback(...perYourAboveCode(err, res))
};

A handler could then look like:

const myHandler = (event, context, awsCallback) => {
  const callback = regularize(awsCallback);
};

answered Mar 3, 2017 at 12:04

bebbi

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