await-promises.js

import fs from 'fs';

function oldStyleAsynchronousFunction(callback) {
    fs.readFile('somefile.txt', 'utf8', function(err, result) {
        if (err) {
            callback(err, null);
            return;
        }

        callback(result);
    });
}

function functionReturningPromise() {
    return new Promise((resolve, reject) => {
        try {
            oldStyleAsynchronousFunction((err, result) =>
                err !== null ? reject(err) : resolve(result)
            );
        } catch (e) {
            reject(e);
        }
    });
}

async function myAsync() {
    /* Anything that returns a promise can be "awaited" */
    let a = await functionReturningPromise();
    return a;
}

/* Awaiting is the same as doing this */
function functionThatUsesPromise() {
    return functionReturningPromise().then(() => someOtherThing());
}

/* What happens if you have multiple awaits in a single function? */
function myAsyncLotsOfAwaits() {
    let a = await functionReturningPromise();
    let b = await functionReturningPromise(a);
    let c = await functionReturningPromise(c);
    
    return c;
}

/* It is the same thing as doing this */
function functionThatUsesPromiseChain() {
    return functionThatUsesPromise.then(a =>
        functionThatUsesPromise(a)
    ).then(b =>
        functionThatUsesPromise(b)
    ).then(c =>
        functionThatUsesPromise(c)
    );
}