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Discover how to utilize the RxJS `pairwise` operator to access the previous value of a Subject in your TypeScript application.
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How to Get the Previous Value of RxJS Subject: A Comprehensive Guide

When working with RxJS in TypeScript, encountering scenarios where you need to compare values is common. One key problem that developers face is how to obtain the previous emitted value of an RxJS Subject or BehaviorSubject. This capability is crucial when making decisions based on the changes in data over time - for example, when adjusting your application’s behavior based on a paginator's pageSize value.

In this guide, we will explore a scenario where you may need to compare the new pageSize value with the previous one, and we will provide a solution using the RxJS pairwise operator.

The Problem: Comparing Previous and Current Values

Imagine you're developing a feature that requires you to load data based on the user's current selection of items per page (i.e., pageSize). In such a case, you may want to check if the new pageSize is larger than the previous pageSize. If it is, you would load new data; if not, no action should be taken.

Here's an example of how this problem might look in TypeScript code:

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

This code raises the question: How can you access the previous emitted value of a RxJS Subject or BehaviorSubject?

The Solution: Using the pairwise Operator

The answer lies in utilizing the pairwise operator from RxJS, which allows you to emit the previous and current values of an Observable as an array. By doing so, you can easily compare the two values within your logic.

Step-by-Step Implementation

Here’s how you can implement the solution:

Import the Required Operators: Ensure that you import pairwise, map, tap, and switchMap from RxJS.

Implement the pairwise Operator: Incorporate the pairwise operator into your observable pipeline where you want to assess the values.

Here is the revised code:

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

Key Points to Remember

BehaviorSubject: This type of Subject always retains the last emitted value, which is useful for scenarios where previous values matter.

pairwise Operator: This operator emits an array consisting of the last two emitted values, making it simple to compare previous and current states.

Logical Decisions: Always ensure that your logic correctly reflects the intended actions based on the comparison of the values.

Conclusion

By employing the pairwise operator, you can efficiently derive the previous value of a RxJS Subject or BehaviorSubject in your TypeScript applications. This technique not only simplifies your logic but also enhances your ability to create responsive and performant features in your applications.

Now you have a powerful tool in your RXJS toolkit to manage state changes effectively. Happy coding!