Deep Dive Into The RxJs mergeMap Operator: How Does it Work?
2 min readDec 30, 2023
The mergeMap
operator in RxJS is a powerful and commonly used operator for transforming the items emitted by an observable into other observables, and then flattening those observables into a single observable sequence. It's often referred to as a "flatMap" operator as well.
Here’s a deep dive into how mergeMap
works:
Basic Syntax:
import { of } from 'rxjs';
import { mergeMap } from 'rxjs/operators';
const source = of('a', 'b', 'c');
source.pipe(
mergeMap(value => /* Observable or Promise */)
).subscribe(result => console.log(result));
How it Works:
Mapping Function:
- For each item emitted by the source observable (
source
in the example), themergeMap
operator applies a mapping function. - The mapping function takes the current value emitted by the source observable and returns a new observable or a promise.
Flattening:
mergeMap
then subscribes to the observable or waits for the promise to resolve.- If the mapping function returns an observable,
mergeMap
flattens it by merging its emissions into the output observable. - If the mapping function returns a…