Conditionally replacing one http observable with another

Question

I want to make one HTTP call, and then based on its results possibly replace it with another one. This is Angular.

this.http.get('foo.txt')
  .map(response => response.text())
  .if(text => text === "TRYAGAIN", 
    this.http.get('bar.txt').map(response => response.text()))
  .subscribe(text => console.log("got text", text);

But although there is an if operator, it does not seem to do what I want here.

I considered (mis)using errors to do this:

this.http.get('foo.txt')
  .map(response => response.text())
  .map(text => {
    if (text === "TRYAGAIN") throw "";
    return text;
  })
  .catch(err => this.http.get('bar.txt').map(response => response.text()))
  .subscribe(text => console.log("got text", text);

But this seems less than ideal as well. What is the correct idiom for handling this kind of situation?

Answer 1

You should use mergeMap (rxjs5) or flatMap (rxjs)

this.http.get('foo.txt')
  .map(response => response.text())
  .mergeMap(text => (text === 'TRYAGAIN' ?
    this.http.get('bar.txt').map(response => response.text())) :
    Observable.of(text))
  .subscribe(...)