Is it redundant to use flatMap in RXJS?

Question

Is it redundant to use flatMap in RXJS?

I recently came across an enlightening article on RXJS which delves into the concept of flatMap.
After understanding its purpose - to flatten observable of observables into a single observable sequence (similar to SelectMany in C#) - I noticed an interesting use case where flatMap was employed for a jQuery getJson request:

var responseStream = requestStream
  .flatMap(function(requestUrl) {
    return Rx.Observable.fromPromise(jQuery.getJSON(requestUrl));
  });

While this usage is common and makes sense, it got me thinking, could it be achieved with a simpler approach?: (The code below doesn't function correctly, but it seems like it should logically work.)

var responseStream = requestStream
  .map(function(requestUrl) {
    return jQuery.getJSON(requestUrl);
  });

?

Question

What exactly sets apart these two code snippets?

Fiddle

javascript rxjs flatmap

Answer 1

Answer №1

The code in the second snippet is not functioning as expected because it generates an Observable of promises rather than values. The `map` operator does not unwrap promises that are returned from it.

Therefore, you still require the `flatMap` or a similar operator to unwrap promises. However, you can directly return a promise within it and simplify the code like this:

var responseStream = requestStream
  .flatMap(function(requestUrl) {
    return jQuery.getJSON(requestUrl);
  });

Answer 2

The code in the second snippet is not functioning as expected because it generates an Observable of promises rather than values. The `map` operator does not unwrap promises that are returned from it.

Therefore, you still require the `flatMap` or a similar operator to unwrap promises. However, you can directly return a promise within it and simplify the code like this:

var responseStream = requestStream
  .flatMap(function(requestUrl) {
    return jQuery.getJSON(requestUrl);
  });

Answer 3

Answer №2

There are multiple questions to address in this situation:

Although the support for promises is currently available, it might not be officially documented and could potentially be removed in the future. To ensure compatibility, consider wrapping every Promise with Observable.fromPromise or a similar method.
It's important to note that internally, flatMap may convert Promises into Observables while map does not. This means that your code snippet may not work as expected because Observables are lazy and require subscription after using the map function to actually execute the Promise.

Answer 4

There are multiple questions to address in this situation:

Although the support for promises is currently available, it might not be officially documented and could potentially be removed in the future. To ensure compatibility, consider wrapping every Promise with Observable.fromPromise or a similar method.
It's important to note that internally, flatMap may convert Promises into Observables while map does not. This means that your code snippet may not work as expected because Observables are lazy and require subscription after using the map function to actually execute the Promise.

Is it redundant to use flatMap in RXJS?

Answer №1

Answer №2

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