Promises combine asynchronous execution with callbacks! Promises run asynchronously and have optional success and failure function arguments.

Async

async causes a function to return a promise. For example, the following functions are equivalent:

async function myFunc() {
    return 'Daltie';
}

async function myFunc2() {
    // return Promise.reject('Bad Daltie!'); // Call error function
    return Promise.resolve('Daltie'); // Call success function
}

myFunc().then(value =< console.log(value));
myFunc2().then(
    value => console.log(value),
    error => console.log(error)
);

// Output
// Daltie
// Daltie

Await

await is used inside of an async function. await waits for Promise to be fulfilled and returns the results.

Examples here.

Chaining

Promises can be chained:

async function myFunc() {
    return 'Apples!';
}

let success = function (value) {
    console.log(value)
    return value;
}

myFunc()
    .then(success)
    .then(success)
    .then(success)
    .catch(x => console.log(x))

// Output:
// Apples!
// Apples!
// Apples!

Syntax

Basic syntax:

let myPromise = new Promise(success_func, failure_func) {
    // Do stuff, call func(s)
}
myPromise.then(
    function(value) { /* Code if successful */ }.
    function(error) { /* Code if some error occurs */ }
);

Both the successful and error function are optional.

Basic example:

// Wait 3 seconds, then call myResolve()
let firstPromise = new Promise(function(myResolve, myReject) {
    setTimeout(function() { myResolve('Yay!'); }, 3000);
});

firstPromise.then(function(value) {
    console.log(value);
})

// Second promise. Will finish first
let secondPromise = new Promise(function(success, failure) {
    if(true) {
        success('Success');
    } else {
        failure('Failure');
    }
});

secondPromise.then(
    function(value) {console.log(value);},
    function(error) {console.log(error);}
);

// Output:
// Success
// Yay!