JS: Promise Tutorial

(new in ECMAScript 2015)

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

Promise.all

Promise.race

Promise.reject

Promise.resolve

Promise.prototype

Promise.prototype.constructor

• JS: Promise.prototype.constructor

Promise.prototype.catch

Promise.prototype.then