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Discover how to effectively join two asynchronous object arrays from different APIs using `RxJs`. Learn step-by-step techniques for managing observable data.
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Merging Asynchronous Values in RxJs: A Complete Guide to Joining Observables

In the world of modern web development, handling asynchronous data efficiently is crucial. When working with Angular and TypeScript, one might often encounter scenarios where data is fetched from multiple API endpoints. This leads to a common challenge: How do you merge multiple object arrays obtained through different asynchronous operations?

In this guide, we’ll address a specific use case of this problem. We’ll explore how to merge data from two different observables, each representing an array of objects fetched from different APIs. Let’s dive into the solution step by step!

The Problem

You're given two observable streams:

firstValues$ - an observable that fetches an array of objects from one API.

secondApi.fetch - a function that retrieves an object from another API, based on each ID obtained from firstValues$.

The goal is to combine these two arrays such that each item from the first array is paired with information from the second. However, they're fetched asynchronously, which complicates the merging process.

The Solution

Step 1: Understanding Observable Streams

Before we begin coding, let’s clarify what we need to do with our observables:

Fetch data concurrently: We want to execute the second API requests concurrently instead of sequentially to improve performance.

Merge both sets of data: After retrieving the object arrays, we will need to combine them into a single usable structure.

Step 2: Using RxJs Operators

To achieve our goal, we’ll utilize several essential RxJs operators, namely:

switchMap(): For flattening inner observables.

forkJoin(): To handle multiple observable results and wait for all of them to complete.

map(): To transform data as needed.

Sample Code

Here's how we can implement this in code:

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

Breakdown of the Code

Fetching First Values:

We initiate the observable chain with this.firstValues$, which emits an array of first values.

Using forkJoin:

Inside switchMap, we take firstValues and create a new array where we call this.secondApi.fetch for each firstValue ID.

forkJoin is then used to wait for all these API calls to complete before continuing.

Mapping the Results:

For each response from the second API, we create a new object that combines firstValue and secondValue.

Final Mapping and Merging:

Finally, we merge the data using our defined method fillFirstWithSecond, which processes the combined results.

Conclusion

Merging asynchronous values in RxJs can initially seem daunting, but by leveraging its powerful operators, you can efficiently handle and combine data streams seamlessly. The approach demonstrated here ensures you make the most of both parallel processing and data transformation capabilities.

With this understanding, you can confidently integrate multiple API responses into cohesive data structures, enhancing both performance and user experience in your Angular applications.

Feel free to reach out if you have any more questions about RxJs or any related topics!