A fake observable

fake-observable.js

let currentCaller = null;

class FakeObservable {
  constructor() {
    this._value = null;
    this._callers = new Set();
  }

  set(val) {
    this._value = val;
    this._callers.forEach(cb => cb());
  }

  get() {
    this._callers.add(currentCaller);
    return this._value;
  }
}

function autorun(callback) {
  if (typeof callback !== 'function') return;

  currentCaller = callback;
  callback();
}

/********* TEST *********/

const obs = new FakeObservable();

autorun(() => {
  console.log('value updated:', obs.get());
});

// log: null

obs.set(1); // log: 1
obs.set(2); // log: 2
obs.set(666); // log: 666