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

To use the mergeMap operator from the rxjs library in JavaScript, first you need to import it from rxjs/operators. Assuming you have already installed rxjs package, it can be done like this:

index.tsx
import { mergeMap } from 'rxjs/operators';
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Then you can use the mergeMap operator in a chain of Observable operators. mergeMap is a transformation operator that maps each value emitted by an Observable into another Observable, and then flattens the result by merging all the output Observable streams into a single stream. Here is an example that shows how to use mergeMap to make an HTTP request for each value emitted by an input Observable:

index.tsx
import { from } from 'rxjs';
import { mergeMap } from 'rxjs/operators';
import { ajax } from 'rxjs/ajax';

const urls = [
  'https://jsonplaceholder.typicode.com/posts/1',
  'https://jsonplaceholder.typicode.com/comments/1',
  'https://jsonplaceholder.typicode.com/albums/1'
];

const requests$ = from(urls).pipe(
  mergeMap(url => ajax.getJSON(url))
);

requests$.subscribe(response => console.log(response));
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In this example, from(urls) creates an input Observable that emits each URL in the urls array. Then mergeMap is used to map each URL into an Observable that makes an HTTP getJSON request using the ajax helper function from rxjs/ajax. The resulting Observable emits the responses obtained from all HTTP requests to each URL. Finally, the subscribe method is used to log each response to the console.