Working with asynchronous functions

async-parallel-right.js

var db = {
    // Async function 1
    findUsers: function (callback) {
        setTimeout(function () {
            console.log('findUsers()');

            callback([
                {name: 'Jacob', age: 22},
                {name: 'Michaela', age: 21}
            ])
        }, 1000)
    },

    // Async function 2
    findPosts: function (callback) {
        setTimeout(function () {
            console.log('findPosts()');

            callback([
                {title: 'First post'},
                {title: 'Last post'}
            ])
        }, 2000)
    }
};

// ---------------------------------------------------------------- //
// Async functions run in PARALLEL, right version
// Both findPosts() and findUsers() start `at the same` time and
// on their finish, we decrement `counter` and if it's `0`, we know that
// both of them finished and we have our values in `parallelRight` object
// Time: 2000 ms
var parallelRight = {};
var counter       = 2; // We have 2 async functions

function isFinished() {
    if (--counter) return; // Decrement by 1, because when this runs, async function invoked callback

    // Both async functions finished
    console.log(parallelRight);
    console.timeEnd('Async Parallel Right');
}

console.time('Async Parallel Right');

db.findPosts(function (posts) {
    parallelRight.posts = posts; // callback body executed after 1000ms
    isFinished()
});
db.findUsers(function (users) {
    parallelRight.users = users; // callback body executed after 2000ms
    isFinished()
});

async-parallel-wrong.js

var db = {
    // Async function 1
    findUsers: function (callback) {
        setTimeout(function () {
            console.log('findUsers()');

            callback([
                {name: 'Jacob', age: 22},
                {name: 'Michaela', age: 21}
            ])
        }, 1000)
    },

    // Async function 2
    findPosts: function (callback) {
        setTimeout(function () {
            console.log('findPosts()');

            callback([
                {title: 'First post'},
                {title: 'Last post'}
            ])
        }, 2000)
    }
};

// ---------------------------------------------------------------- //
// Async functions run in PARALLEL, wrong version
// Both findPosts() and findUsers() start `at the same` time, but
// `console.log(parallelWrong)` prints before they have chance to finish
// Time: 2000 ms
var parallelWrong = {};

console.time('Async Parallel Wrong');

db.findPosts(function (posts) {
    parallelWrong.posts = posts; // callback body executed after 1000ms
});
db.findUsers(function (users) {
    parallelWrong.users = users; // callback body executed after 2000ms
});
console.log(parallelWrong); // Prints {}

console.timeEnd('Async Parallel Wrong');

async-series.js

var db = {
    // Async function 1
    findUsers: function (callback) {
        setTimeout(function () {
            console.log('findUsers()');

            callback([
                {name: 'Jacob', age: 22},
                {name: 'Michaela', age: 21}
            ])
        }, 1000)
    },

    // Async function 2
    findPosts: function (callback) {
        setTimeout(function () {
            console.log('findPosts()');

            callback([
                {title: 'First post'},
                {title: 'Last post'}
            ])
        }, 2000)
    }
};

// ---------------------------------------------------------------- //
// Async functions run in SERIES
// First runs findPosts(), when it's callback is executed, findUsers() is invoked
// Time: 3000 ms (actually always a bit more) TODO reference
var series = {};

console.time('Async Series');

db.findPosts(function (posts) {
    db.findUsers(function (users) {
        series.posts = posts;
        series.users = users;
        console.log(series);

        console.timeEnd('Async Series');
    });
});