fourscience

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);
      })
    );
  }