Common Node.js mistakes in Lambda

October 22, 2019 0 Comments

Common Node.js mistakes 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.

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 

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.

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 { team, 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 { team, 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 = .map(teamIds, id => await teamModel.fetch(id)) return teams 

Instead, you should write it as the following:

async function getTeams(teamIds) { const promises = .map(teamIds, 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.

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(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!

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!


That's it, 5 common mistakes to avoid when working with Node.js in Lambda. Follow me for more tips on building production-ready serverless applications and operational best practices.

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