https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise

The Promise object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value.

Essentially, a promise is a returned object to which you attach callbacks, instead of passing callbacks into a function.

let promise = doSomething(); 

promise.then(successCallback, failureCallback);

or simply:

doSomething().then(successCallback, failureCallback);

We call this an asynchronous function call. This convention has several advantages. We will explore each one.

Guarantees

Unlike old-style passed-in callbacks, a promise comes with some guarantees:

• Callbacks will never be called before the completion of the current run of the JavaScript event loop.
• Callbacks added with.theneven after the success or failure of the asynchronous operation, will be called, as above.
• Multiple callbacks may be added by calling.thenseveral times, to be executed independently in insertion order.

But the most immediate benefit of promises is chaining.

function getRandomInt(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(Math.random() * (max - min)) + min; //The maximum is exclusive and the minimum is inclusive
}


function get() {
    return getRandomInt(0, 61) % 2 === 0 ? true : false;
}


let promiseIsEven = new Promise(function(resolve, reject) {
    if(get()) {
        resolve('success ' + new Date().getMilliseconds());
    } else {
        reject('failure ' + new Date().getMilliseconds());
    }
});

promiseIsEven.then(function(fromResolve) {
    console.log(fromResolve);
}).catch(function(fromReject) {
    console.log(fromReject);
});