how to use the expand function from the rxjs library in javascript

The expand operator in the rxjs library is used to recursively apply a projection function to each value emitted by a source observable, while ensuring that a maximum depth is not exceeded. Here's an example of how to use it:

index.tsx
const { of } = require('rxjs');
const { expand, take } = require('rxjs/operators');

// Define the projection function
const projectionFn = x => of(x + 1);

// Define the source observable
const source$ = of(1);

// Use the expand operator
const final$ = source$.pipe(
  expand(projectionFn),
  take(5) // Prevent an infinite loop by specifying a maximum depth
);

// Subscribe to the final observable
final$.subscribe(console.log);
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In this example, we first import the of, expand, and take operators from rxjs. We then define the projectionFn function, which takes an input value and returns an observable that emits a value derived from the input.

Next, we create a source observable source$ that emits the value 1. We then use the expand operator to recursively apply the projectionFn to each emitted value, thus creating a new observable that emits a stream of values starting from 1 and continuously adding 1 to each subsequent value. We use the take operator to ensure that the recursion is stopped after the fifth value is emitted.

Finally, we subscribe to the final$ observable and log its values to the console. The output will be:

index.tsx
1
2
3
4
5
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indicating that the expand function has applied the projectionFn function iteratively to each emitted value until the maximum depth of 5 was reached.