Скачать или смотреть Understanding mergeMap vs switchMap: Why Isn't mergeMap Accumulating Results?

Explore the differences between `mergeMap` and `switchMap` in RXJS to understand their behavior in observable streams, especially when handling multiple inner observables.
---
This video is based on the question https://stackoverflow.com/q/72858166/ asked by the user 'Laurence Fass' ( https://stackoverflow.com/u/1236657/ ) and on the answer https://stackoverflow.com/a/72858914/ provided by the user 'Mrk Sef' ( https://stackoverflow.com/u/13500986/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: RXJS: Why is mergemap not providing output from multiple inner observables in example?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding mergeMap vs switchMap: Why Isn't mergeMap Accumulating Results?

If you're working with Reactive Programming in JavaScript using RXJS, you might have stumbled upon a situation where you expect mergeMap to accumulate results from multiple inner observables, but instead, you're seeing behavior that resembles switchMap. This post will dive deeper into this topic and clarify the nuances between these two operators in RXJS.

The Problem

As a developer exploring RXJS, you may expect that mergeMap can collect results from multiple inner observables as you interact with your application, such as clicking on a document or performing an action. However, you might encounter a scenario where each successive document click only outputs a single result rather than accumulating multiple results as anticipated.

A Sample Code Snippet

[[See Video to Reveal this Text or Code Snippet]]

In the example above, the expectation is that every click will provide a unique timestamp, accumulating them together. Instead, the results behave similarly to switchMap, where each new click appears to discard the previous observables.

Understanding mergeMap Behavior

What Does mergeMap Do?

To clarify this confusion, let's break down how mergeMap really functions:

Creates a Strong Merge: mergeMap doesn't mean that it will merge the emitted items from different streams; rather, it means it merges these streams together. Each time it encounters a new source value, it creates a new inner observable and subscribes to it independently.

Single Emission with Completion: In the case of of(value), which emits a single value and then completes, it does not provide continued emissions over time. When this observable is used within mergeMap, every click creates a new observable that emits a value, but since of completes quickly, you don’t see those emissions accumulate over different clicks.

An Experiment to Differentiate

To see the differences in action, let’s try modifying the example using a slow-fetch simulation that provides multiple values:

[[See Video to Reveal this Text or Code Snippet]]

Observing the Difference

Using mergeMap: Each click will result in a new subscription to getDate that remains active independently, causing multiple timestamps to be logged to the console as each observable completes after 3 seconds.

Using switchMap: In contrast, if you try switchMap, only the most recent click will be active. Older click events will be canceled in favor of new subscriptions, leading to a dropped observable.

The Key Takeaway

The confusion often arises because of(value) emits and completes swiftly, resulting in no observable accumulation. To truly see the accumulation behavior you expect from mergeMap, you would have to use an observable capable of emitting multiple values over time - such as interval().

For example:

[[See Video to Reveal this Text or Code Snippet]]

This will provide outputs for both streams, wherein the interval will continue emitting values without completing immediately.

Final Thoughts

In summary, mergeMap is powerful when you expect to manage multiple streams simultaneously. It's essential to choose your inner observable wisely to utilize the merging ability effectively. If you find yourself needing results to gather and not just be replaced, your inner observable should emit over time rather than completing immediately.

If you're interested in further exploring RXJS, consider implementing a resubscribingMergeMap operator that will allow resubscriptions on every new source observable emission to investigate its different behaviors further. Happy coding!