Promise in Javascript

Description
A Promise is a proxy for a value not necessarily known when the promise is created. It allows you to associate handlers with an asynchronous action's eventual success value or failure reason. This lets asynchronous methods return values like synchronous methods: instead of immediately returning the final value, the asynchronous method returns a promise to supply the value at some point in the future.

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A Promise is in one of these states:

While a Promise object is "pending" (working), the result is undefined.
When a Promise object is "fulfilled", the result is a value.
When a Promise object is "rejected", the result is an error object.

myPromise.state myPromise.result :
"pending" -> undefined
"fulfilled" -> a result value
"rejected" -> an error object

Exmample :

[Promises](function myDisplayer(some) {
  document.getElementById("demo").innerHTML = some;
}

let myPromise = new Promise(function(myResolve, myReject) {
  let x = 0;

// The producing code (this may take some time)

  if (x == 0) {
    myResolve("OK");
  } else {
    myReject("Error");
  }
});

myPromise.then(
  function(value) {myDisplayer(value);},
  function(error) {myDisplayer(error);}
);)