Combining the results of an outer & inner observable into a single parameter (array) for the next map method

Question

What I'm trying to do is similar to combineLatest, but the second (inner) observable depends on the response from the first

ajax.get('/first-url')
  .map(res => new Thing(res.response)
  .someMap(thing => 
    ajax.get(`/second-url?code=${thing.code}`)
      .map(res => new OtherThing(res.response))
  ).map(([firstThing, secondThing]) => console.log(firstThing, secondThing))

Essentially, I'd like to be able to merge the two results into a single array of those results that I can use to do a third thing.

My goal is to get data A and data B because I need them BOTH to perform a certain action, and getting data B relies on already having data A.

Answer 1

I was able to get this working by doing the following:

ajax.get('/first-url')
  .map(res => new Thing(res.response)
  .mergeMap(firstThing=> 
    ajax.get(`/second-url?code=${firstThing.code}`)
      .map(res => new OtherThing(res.response))
      .map(secondThing => [firstThing, secondThing])
  ).map(([firstThing, secondThing]) => console.log(firstThing, secondThing))

Essentially, in the response from the second observable, return the exact array that I want to consume in the outer observable's .map

It appears that this is rxjs' desired way of handling this. In version 5.x of rxjs, there was a thing called a Result Selector, which could be passed in as a second parameter to create something like this (it would look a lot like the last inner .map) but this appears to have been deprecated for rxjs v6.0:

https://github.com/ReactiveX/rxjs/blob/master/docs_app/content/guide/v6/migration.md#howto-result-selector-migration