Get Difference Between Last Two Values Of An Rxjs Observable

RxJS, short for Reactive Extensions for JavaScript, is a powerful library that allows you to work with asynchronous and event-based programming in a more manageable and streamlined manner. One common scenario you might encounter when working with RxJS observables is the need to calculate the difference between the last two values emitted by an observable. This can be particularly useful when you want to track changes in a sequence of values over time.

To get the difference between the last two values emitted by an RxJS observable, you can leverage the `pairwise` operator provided by RxJS. The `pairwise` operator transforms an observable sequence into a new sequence of paired consecutive values. This allows you to compare the last emitted value with the current one and perform the necessary calculations.

Here's a step-by-step guide on how to achieve this:

1. Import the necessary modules: First, ensure you have RxJS installed in your project. You can import the required modules in your code using the following statement:

import { of } from 'rxjs';
import { pairwise } from 'rxjs/operators';

2. Create an observable: Create an observable that emits a sequence of values over time. For demonstration purposes, let's consider a simple observable that emits incremental numbers every second.

const observable = of(1, 2, 3, 5, 8, 13);

3. Apply the pairwise operator: Pipe the `pairwise` operator to your observable to transform it into a paired sequence of values.

observable.pipe(pairwise())
    .subscribe(pair => {
        const [previous, current] = pair;
        const difference = current - previous;
        console.log(`Difference between ${current} and ${previous} is ${difference}`);
    });

4. Run the code: When you run the code, the `pairwise` operator will pair consecutive values emitted by the observable, allowing you to calculate the difference between the last two values and log the result to the console.

In this example, the `pairwise` operator pairs values in the observable sequence like this:
- Pair 1: (1, 2)
- Pair 2: (2, 3)
- Pair 3: (3, 5)
- Pair 4: (5, 8)
- Pair 5: (8, 13)

By subtracting the previous value from the current value in each pair, you can easily calculate the difference between the last two values. This technique can be extremely useful in scenarios where you need to track changes or deltas in your data stream.

By following these simple steps and leveraging the `pairwise` operator in RxJS, you can efficiently calculate the difference between the last two values emitted by an observable. This approach enhances your ability to analyze data streams and make informed decisions based on the changes in values over time.