Gergely Nemeth Co-Founder of RisingStack, EM at Uber

A couple of months ago async/await landed in V8, the JavaScript engine. In the meantime, V8 was updated multiple times in Node.js, and the latest nightly build finally added the V8 version that supports the async/await functionality to Node.js.

Disclaimer: the async/await functionality is only available in the nightly, unstable version of Node.js. Do not use it in production for now.

What's async/await?

First, let's see how you are doing async operations with Promises! This little example shows you how you can fetch data using the Fetch API and Promises.

function getTrace () { return fetch('https://trace.risingstack.com', { method: 'get' }) } getTrace() .then() .catch()

With async/await, you can await on Promises. This will halt the execution in a non-blocking way - since it waits for the result and returns it. If the promise is not resolved but rejected, the rejected value will be thrown, meaning it can be caught with a try/catch block.

The previous example rewritten with async/await would look something like this:

async function getTrace () { let pageContent try { pageContent = await fetch('https://trace.risingstack.com', { method: 'get' }) } catch (ex) { console.error(ex) } return pageContent } getTrace() .then()

For more information on async/await, I recommend reading the following resources:

Using async/await without transpilers

Installing Node 7

To get started, you have to get the latest build of Node.js first. To do so, head over to the Nightly builds and grab the latest one of the v7.

Once you have downloaded it, unpack it - and you are ready to use it!

If you are using nvm, you can try to install it this way:

NVM_NODEJS_ORG_MIRROR=https://nodejs.org/download/nightly nvm install 7 nvm use 7

Running files with async/await

Let's create a simple JavaScript file that delays the execution of a function using the setTimeout call, but wrapped with async/await calls.

// app.js const timeout = function (delay) { return new Promise((resolve, reject) => { setTimeout(() => { resolve() }, delay) }) } async function timer () { console.log('timer started') await Promise.resolve(timeout(100)); console.log('timer finished') } timer()

Once you have this file, you could try running with:

node app.js

However, it won't work. The async/await support is still behind a flag. To run it, you have to use:

node --harmony-async-await app.js

Building a web server with async/await

As of Koa v2, Koa supports async functions as middlewares. Previously, it was only possible with transpilers, but from now on it is not the case!

You can simply pass an async function as a Koa middleware:

// app.js const Koa = require('koa') const app = new Koa() app.use(async (ctx, next) => { const start = new Date() await next() const ms = new Date() - start console.log(`${ctx.method} ${ctx.url} - ${ms}ms`) }) app.use(ctx => { ctx.body = 'Hello Koa' }) app.listen(3000)

Once you have a working server written using Koa, you can simply start it with:

node --harmony-async-await app.js

When to start using it?

Node.js v8, the next stable version containing the V8 version that enables async/await operations will be released in April 2017. Till that time you can still experiment with it using the unstable Node.js v7 branch.