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Struggling with RxJS observables in NestJS? Discover how to manage asynchronous code correctly and avoid returning empty results in your functions.
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How to Handle RxJS Observables in NestJS: A Guide to Managing Asynchronous Code

When working with server streaming in gRPC using NestJS, you might run into issues with RxJS observables, specifically when trying to collect values into an array. This is a common problem for developers who are trying to mix synchronous and asynchronous code. In this guide, we will dissect this issue and provide a clear solution.

The Problem

You may encounter a situation where a function seems to return an empty array immediately, even though values are being logged correctly when subscribing to an observable. Here's a glimpse of the code that often leads to confusion:

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

What's Going Wrong?

The root of the problem lies in the fact that the subscribe method handler (next, error, and complete) executes asynchronously. This means your findAllRepos function returns the results array before any values have been pushed into it.

In a synchronous environment like JavaScript, attempting to operate with asynchronous code can lead to frustrating experiences, where you expect one result but get another due to timing issues.

A Common Example

For a better understanding, consider the following example:

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

In this case, the infinite loop prevents the setTimeout from executing. You can see how control flows in asynchronous operations are managed by the JavaScript runtime, and trying to force synchronous behavior can lead to deadlocks and other issues.

The Solution

The best practice in such scenarios is to utilize promises or observables effectively. This allows you to handle asynchronous operations and return results in a way that respects their asynchronous nature.

To resolve the initial problem, change the return type of findAllRepos to return an Observable rather than a synchronous array:

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

Subscribing to the Result

To get the actual value from the observable elsewhere in your application, you can subscribe to it as follows:

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

By implementing this strategy, you ensure that your code handles the asynchronous nature of RxJS observables properly, avoiding the pitfall of returning empty results.

Conclusion

In summary, managing asynchronous code with RxJS can initially be challenging due to the way observables operate. However, with understanding and the strategies outlined in this post, you can effectively handle server streaming in NestJS and ensure that your functions behave as expected. Remember, returning observables instead of direct results is key to success when working with asynchronous data streams.