为何RxJS switchMap返回普通数组时无需传入Observable？NgRx Effects场景下的技术疑问

Great question! This is one of those handy RxJS quirks that catches a lot of developers off guard at first—let me break down why both approaches behave identically.

First, let's look at how switchMap operates under the hood. The projection function you pass to switchMap doesn’t only accept an Observable as its return value. It accepts something called an ObservableInput, a type that includes:

• Observables
• Arrays (and other iterable values)
• Promises

RxJS automatically converts any ObservableInput into an Observable for you. So when you return an array like [val, val + 1] directly in switchMap, RxJS does the exact same work as if you wrapped it with from(): it creates an Observable that emits each element of the array one after another.

Let’s confirm with your examples:

1. Using from() explicitly:

   timer(1000).pipe(switchMap(val => from([val, val + 1]))).subscribe(val => console.log(val));
   

   Here, from() turns the array into an Observable that emits val, then val + 1 in sequence.

2. Returning the array directly:

   timer(1000).pipe(switchMap(val => [val, val + 1])).subscribe(val => console.log(val));
   

   Here, switchMap recognizes the array as an ObservableInput, internally converts it to an Observable (just like from() does), and emits the exact same values in the same order.

For NgRx Effects specifically, this is super useful when you need to dispatch multiple actions from a single Effect. Whether you return from([Action1, Action2]) or just [Action1, Action2], RxJS will handle converting it into a stream of actions that the store can process.

A quick note on readability: while both are valid, using from() can make your intent clearer for other developers who might not be familiar with this RxJS behavior. But functionally, there’s no difference between the two approaches.

内容的提问来源于stack exchange，提问作者Vash42