JS: Promise Tutorial

New in JS2015.

2017-04-06 this page is work in progress.

A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly asynchronous) computation.

// define a asynchronous function using Promise object

const af =  new Promise ( function (f1, f2) { f1 ("f1 called") } );

// call it

af.then ( x => console.log( x ));

// prints "f1 called"

Any Promise object is in one of three mutually exclusive states: fulfilled, rejected, and pending:

• A promise p is fulfilled if p.then(f, r) will immediately enqueue a Job to call the function f.
• A promise p is rejected if p.then(f, r) will immediately enqueue a Job to call the function r.
• A promise is pending if it is neither fulfilled nor rejected.
• A promise is said to be settled if it is not pending, i.e. if it is either fulfilled or rejected.

A promise is resolved if it is settled or if it has been “locked in” to match the state of another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending, fulfilled or rejected.

new Promise(f)

f should be a function that takes 2 args, resolveF, rejectF. Both are also functions.

const pp = new Promise (
    (g1, g2) => {
        setTimeout ( function () { g1("g1 called"); }, 999);
    }
);

pp.then ( (x) => {
    // x is whatever we passed in the g1() function
    console.log( x);
});

// prints "g1 called"

Properties

1. Promise.prototype
2. Promise.all
3. Promise.race
4. Promise.reject
5. Promise.resolve

Promise.prototype

Promise.all

Promise.race

Promise.reject

Promise.resolve

1. Promise.prototype.constructor
2. Promise.prototype.catch
3. Promise.prototype.then

Promise.prototype.constructor

Promise.prototype.catch

Promise.prototype.then