Use Async or Promise in AWS Lambda Handler

In AWS lambda, before node.js runtime 8.10, although you can use promise across all your code, but in top handler, you must follow the below callback pattern. async/await is not supported in handler, even in other functions you must use something like babel to transform your code before deploy.

Previous callback pattern

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const api = require('something');

exports.handler = (event, context, callback) => {
  const getSomthingPromise = api.getSomthing(event);
  getSomthingPromise.then(
    data => {
      callback(null, data);
    },
    err => {
      console.log(err);
      callback(err);
    }
  );
};

With the new Node.js 8.10, you can use promise or async/await pattern in handler, just like normal node.js function. And you don’t need to pass context and callback parameters, only event.

Use async/await

for sync value without await:

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exports.handler = async (event) => {
  return {
    statusCode: 200,
    body: 'ok'
  }
}

for async value with await:

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const api = require('something');

exports.handler = async (event) => {
  const data = await api.getSomthing(event);
  return {
    statusCode: 200,
    body: JSON.stringify({ data })
  };
};

Use Promise

for sync value wrap in Promise:

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exports.handler = (event) => {
  return Promise.resolve({
    statusCode: 200,
    body: 'ok'
  })
};

for async:

you can directly return it, but I don’t recommend this way

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const api = require('something');

exports.handler = (event) => {
  return api.getSomthing(event);
};

It’s better start from Promise.resolve():

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exports.handler = (event) => {
  return Promise.resolve(event)
    .then(api.getSomthing);
};

Summary

To use async or Promise, it’s up to your choice. But I’m sure either way is easier than the previous callback pattern. Also to mention again, async/await is actually a syntactic sugar for promise in js.

Ref:
https://aws.amazon.com/blogs/compute/node-js-8-10-runtime-now-available-in-aws-lambda/