How to create a function that returns an existing promise instead of new promise?

Question

JavaScript/Promise experts,

I hope you can help me, because I don\'t understand how I can create a function that returns an existing promise instead of a new promise

Answer 1

You'll want to memoise the promise, not the value that it resolves with. Memoisation works fine with promises as result values.

var p = null;
function notSoRandomAsyncNumber() {
  if (!p)
    p = new Promise(function(resolve) {
      setTimeout(function() {
        resolve(Math.random());
      }, 1000);
    });
  return p;
}

Or, abstracted into a helper function:

function memoize(fn) {
  var cache = null;
  return function memoized(args) {
    if (fn) {
      cache = fn.apply(this, arguments);
      fn = null;
    }
    return cache;
  };
}
function randomAsyncNumber() {
  return new Promise(res => {
    setTimeout(() => resolve(Math.random()), 1000);
  });
}
function randomAsyncNumberPlusOne() {
  return randomAsyncNumber().then(n => n+1);
}
var notSoRandomAsyncNumber = memoize(randomAsyncNumber);
var notSoRandomAsyncNumberPlusOne = memoize(randomAsyncNumberPlusOne);

(notice that notSoRandomAsyncNumberPlusOne still will create a randomAsyncNumber() on the first call, not a notSoRandomAsyncNumber())