Basic Await and Async

Likely of ES7, available Edge and Firefox (currently Stage-3).

Async and Await are keywords used in tandem to help make Promises asynchronous and more predictable.

First a quick recap on Promises.

import request from 'request';

function getAddress () {
    return new Promise((resolve, reject) => {
        request('https://anyaddress.com/api/get', (err, res, body) => {
            if (err) {
                reject(err); return;
            }
            resolve(body);
        });
    });
}

We instantiate a new Promise and then resolve or reject dependent on the result from the XHR request.

getAddress()
    .then(data => {
        count : data.length,
        result : data
    })
    .catch(err => console.error(err));

We can then leverage the then and catch method to cleanly deal with results from out XHR request. We avoid callback nesting.

We can also group our promises.

function multiAddresses() {
    return Promise.all([getAddress(), getAddress(), getAddress()]);
}

multiAddresses()
    .then(data => {
        count : data.length,
        results : data
    })
    .catch(err => console.error(err));

This will return all our requests or will catch an error if any fails.

Using Async and Await

async function asyncFunc () {
    const address = await getAddress()
        .then(data => {
            count : data.length,
            result : data
        })
    return address;
}
asyncFunc().then(data => console.log(data.result));

Note that await may only be used in functions marked with the async keyword. Similarly to generators, suspending execution in your context until the promise settles. If the awaited expression isn’t a promise, its casted into a promise.

Chaining Async functions

async function asyncFunc1 () {
    return await asyncFunc2()
        .then(data => data)
}

async function asyncFunc2 () {
    return await getAddress()
        .then(data => {
            count : data.length,
            result : data
        })
}

asyncFunc1().then(data => console.log(data.result));

Async functions will return an Promise, so we can chain async functions and know they will run syncronously. This is very useful if you are relying on the returned results to build future queries.