Top 5 Common Node.js mistakes in Lambda

Top 5 Common Node.js mistakes in Lambda

Top 5 common mistakes to avoid when working with Node.js in Lambda

I have helped quite a few clients with their Node.js serverless projects. In doing so I have seen some recurring mistakes around async/await.

1. Still using callbacks

Many people are still using the callbacks in their async handler functions:

module.exports.handler = async (event, context, cb) => {
  const response = {
    statusCode: 200,
    body: JSON.stringify({ message: 'hello world' })

  cb(null, response)

instead of the simpler alternative:

module.exports.handler = async (event, context) => {
  const response = {
    statusCode: 200,
    body: JSON.stringify({ message: 'hello world' })

  return response

2. Not using promisify

Before Node8, bluebird filled a massive gap. It provided the utility to convert callback-based functions to promise-based. But Node8's built-in util module has filled that gap with the promisify function.

For example, we can now transform the readFile function from the fs module like this:

const fs = require('fs')
const { promisify } = require('util')
const readFile = promisify(fs.readFile)

No need to use bluebird anymore. That's one less dependency, which helps reduce the cold start time for our functions.

3. Too sequential

async/await lets you write asynchronous code as if they're synchronous, which is awesome. No more dealing with callback hell!

On the flip side, we can also miss a trick and not perform tasks concurrently where appropriate.

Take the following code as an example:

async function getFixturesAndTeam(teamId) {
  const fixtures = await fixtureModel.fetchAll()
  const team = await teamModel.fetch(teamId)
  return {
    fixtures: fixtures.filter(x => x.teamId === teamId)

This function is easy to follow, but it's hardly optimal. teamModel.fetch doesn't depend on the result of fixtureModel.fetchAll, so they should run concurrently.

Here is how you can improve it:

async function getFixturesAndTeam(teamId) {
  const fixturesPromise = fixtureModel.fetchAll()
  const teamPromise = teamModel.fetch(teamId)

  const fixtures = await fixturesPromise
  const team = await teamPromise

  return {
    fixtures: fixtures.filter(x => x.teamId === teamId)

In this version, both fixtureModel.fetchAll and teamModel.fetch are started concurrently.

You also need to watch out when using map with async/await. The following will call teamModel.fetch one after another:

async function getTeams(teamIds) {
  const teams =, id => await teamModel.fetch(id))
  return teams

Instead, you should write it as the following:

async function getTeams(teamIds) {
  const promises =, id => teamModel.fetch(id))
  const teams = await Promise.all(promises)
  return teams

In this version, we map teamIds to an array of Promise. We can then use Promise.all to turn this array into a single Promise that returns an array of teams.

In this case, teamModel.fetch is called concurrently and can significantly improve execution time.

4. async/await inside forEach()

This is a tricky one, and can sometimes catch out even experienced Node.js developers.

The problem is that code like this doesn't behave the way you'd expect it to:

[ 1, 2, 3 ].forEach(async (x) => {
  await sleep(x)

console.log('all done.')

When you run this you'll get the following output:

all done.

See this post for a longer explanation about why this doesn't work. For now, just remember to avoid using async/await inside a forEach!

5. Not using AWSSDK’s .promise()

Did you know that the AWS SDK clients support both callbacks and promises? To use async/await with the AWS SDK, add .promise() to client methods like this:

const AWS = require('aws-sdk')
const Lambda = new AWS.Lambda()

async function invokeLambda(functionName) {
  const req = {
    FunctionName: functionName,
    Payload: JSON.stringify({ message: 'hello world' })
  await Lambda.invoke(req).promise()

No more callback functions, yay!

node-js nodejs node aws

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