JS: Promise Tutorial

(new in JS: ECMAScript 2015)

xtodo 2025-06-06 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.

new Promise(executorF)

executorF should be a function that takes 2 args, fResolve, fReject. Both are also functions.

const execF = (fResolve, fReject) => {
  setTimeout(() => {
    fResolve("fResolve called");
  }, 999);
};

const pms = new Promise(execF);

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

// define a asynchronous function using Promise object

const pms = new Promise((f1, f2) => {
  f1("f1 called");
});

// call it
pms.then((x) => console.log(x));

// prints "f1 called"

States of Promise

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

• p is fulfilled if p.then(f, r) will immediately enqueue a Job to call the function f.
• p is rejected if p.then(f, r) will immediately enqueue a Job to call the function r.
• p is pending if it is neither fulfilled nor rejected.

p is settled if it is not pending.

• 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.
• An unresolved promise is always in the pending state.
• A resolved promise may be pending, fulfilled or rejected.

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