// This code is tested
// install node.js and you can run it on command line with: node Signal.js
// it will just say: Hello Alice
// see the main() function on the bottom

export class Pulse {
  #value;
  #subscribers;
  #disposables;
  constructor(value) {
    this.#value = value;
    this.#subscribers = new Set();
    this.#disposables = new Set();
  }

  get value() {
    return this.#value;
  }

  set value(newValue) {
    // IMPORTANT FEATURE: if value is the same, exit early, don't disturb if you don't need to
    if (newValue == this.#value) return;

    this.#value = newValue;
    this.notify(); // all observers
  }

  subscribe(subscriber) {

    // IMPORTANT FEATURE: instant notification (initialization on subscribe), but don't notify on null/undefined
    if (this.#value !== null) subscriber(this.#value);

    this.#subscribers.add(subscriber);

    // IMPORTANT FEATURE: return unsubscribe function, execute this to stop getting notifications.
    return () => this.#subscribers.delete(subscriber);
  }

  notify() {
    for (const subscriber of this.#subscribers) subscriber(this.#value);
  }

  clear() {
    this.#subscribers.clear();
    this.#disposables.forEach((disposable) => disposable());
    this.#disposables.clear();
  }

  dispose(disposable) {
    this.#disposables.add(disposable);
  }
}

export class Signal extends Pulse { // we are making the pulse into a Signal
  map(mapFunctionPredicate) { // here we are wrapping the function with the method
    return map(this, mapFunctionPredicate);
  }
}

export function map(parentSignal, mapFunctionPredicate) {
  const mappedSignal = new Signal(undefined);
  const subscription = parentSignal.subscribe((parentSignalValue) => (mappedSignal.value = mapFunctionPredicate(parentSignalValue)));
  mappedSignal.dispose(subscription);
  return mappedSignal;
}

function main(){
  const usernameSignal = new Signal();
  usernameSignal.value = "Alice";
  const mappedSignal = usernameSignal.map(v=>`Hello ${v}`);
  mappedSignal.subscribe(console.log)
}
main(); // run program