tinyobserver.js

function NOOP(){}
function Observable(subscriber = NOOP) { 
  function create(operand = v => v) {
    return new Observable(observer => this.subscribe(v => {
        let b = operand(v);
        this.value = typeof b !== 'boolean' ? b : b && v;
        this.value && observer.next(this.value);
      })
    );
  }
  
  function subscribe(next, error = NOOP, complete = NOOP) {
    let unsubscribe;
    this.observer = {
      next: v => { this.value = v; next(v) },
      error,
      complete: () => { complete(); unsubscribe() },
    };
    unsubscribe = subscriber(this.observer)
    return { unsubscribe }
  }

  return {
    create,
    subscribe,
    map: create,
    value: null,
    filter: create,
    observer: null,
  }
}