Long Zhao Long Zhao - 29 days ago 7
Javascript Question

RxJS, Observable, how to preserve value and switch map to another one

// ticker$ will update every 3s
// showHand$ will only triger after user click button
// I would like to take last ticker price as user order price when user click button

let lastPrice: number;

this.ticker$
// What I am doing now is preserve value to vairable here.
.do(ticker => lastPrice = ticker.closePrice)
.switchMap(() => this.showHand$)
.subscribe(showHand => {
// use value here
this.order.price = lastPrice;
this.order.amount = showHand.amount;
this.order.type = showHand.type;

this.submit();
});


Any segestion about how to prevser value and switch map together, without one line variable like above?

Answer Source

I think this is the operator

this.showHand$.take(1)
  .withLatestFrom(this.ticker$)
  .subscribe(([showHand, ticker]) => {
    this.order.price = ticker.closePrice;
    this.order.amount = showHand.amount;
    this.order.type = showHand.type;
    this.submit();      
  });

Note, take(1) will close subscription, but if you want the user to be able to press the button many times, save the subscription to a const and unsubscribe when finished.