mergeMap / flatMap

signature: mergeMap(project: function: Observable, resultSelector: function: any, concurrent: number): Observable

Map to observable, emit values.

💡 flatMap is an alias for mergeMap!

💡 If only one inner subscription should be active at a time, try switchMap!

💡 If the order of emission and subscription of inner observables is important, try concatMap!

Why use mergeMap?

This operator is best used when you wish to flatten an inner observable but want to manually control the number of inner subscriptions.

For instance, when using switchMap each inner subscription is completed when the source emits, allowing only one active inner subscription. In contrast, mergeMap allows for multiple inner subscriptions to be active at a time. Because of this, one of the most common use-case for mergeMap is requests that should not be canceled, think writes rather than reads. Note that if order must be maintained concatMap is a better option.

Be aware that because mergeMap maintains multiple active inner subscriptions at once it's possible to create a memory leak through long-lived inner subscriptions. A basic example would be if you were mapping to an observable with an inner timer, or a stream of dom events. In these cases, if you still wish to utilize mergeMap you may want to take advantage of another operator to manage the completion of the inner subscription, think take or takeUntil. You can also limit the number of active inner subscriptions at a time with the concurrent parameter, seen in example 5.

Examples

Example 1: mergeMap simulating save of click locations

( StackBlitz )

// RxJS v6+
import { fromEvent, of } from 'rxjs';
import { mergeMap, delay } from 'rxjs/operators';

// faking network request for save
const saveLocation = location => {
  return of(location).pipe(delay(500));
};
// streams
const click$ = fromEvent(document, 'click');

click$
  .pipe(
    mergeMap((e: MouseEvent) => {
      return saveLocation({
        x: e.clientX,
        y: e.clientY,
        timestamp: Date.now()
      });
    })
  )
  // Saved! {x: 98, y: 170, ...}
  .subscribe(r => console.log('Saved!', r));

Example 2: mergeMap with ajax observable

( StackBlitz )

// RxJS v6+
import { fromEvent } from 'rxjs';
import { ajax } from 'rxjs/ajax';
import { mergeMap } from 'rxjs/operators';

// free api url
const API_URL = 'https://jsonplaceholder.typicode.com/todos/1';

// streams
const click$ = fromEvent(document, 'click');

click$
  .pipe(
    /*
     * Using mergeMap for example, but generally for GET requests
     * you will prefer switchMap.
     * Also, if you do not need the parameter like
     * below you could use mergeMapTo instead.
     * ex. mergeMapTo(ajax.getJSON(API_URL))
     */
    mergeMap(() => ajax.getJSON(API_URL))
  )
  // { userId: 1, id: 1, ...}
  .subscribe(console.log);

Example 3: mergeMap with promise (could also use from to convert to observable)

( StackBlitz )

// RxJS v6+
import { of } from 'rxjs';
import { mergeMap } from 'rxjs/operators';

// helper to create promise
const myPromise = val =>
  new Promise(resolve => resolve(`${val} World From Promise!`));

// emit 'Hello'
const source$ = of('Hello');

// map to promise and emit result
source$
  .pipe(mergeMap(val => myPromise(val)))
  // output: 'Hello World From Promise'
  .subscribe(val => console.log(val));

Example 4: mergeMap with resultSelector

( StackBlitz )

// RxJS v6+
import { of } from 'rxjs';
import { mergeMap } from 'rxjs/operators';

// helper to create promise
const myPromise = val =>
  new Promise(resolve => resolve(`${val} World From Promise!`));

// emit 'Hello'
const source$ = of('Hello');

source$
  .pipe(
    mergeMap(
      val => myPromise(val),
      /*
      you can also supply a second argument which receives the source value and emitted
      value of inner observable or promise
    */
      (valueFromSource, valueFromPromise) => {
        return `Source: ${valueFromSource}, Promise: ${valueFromPromise}`;
      }
    )
  )
  // output: "Source: Hello, Promise: Hello World From Promise!"
  .subscribe(val => console.log(val));

Example 5: mergeMap with concurrent value

( StackBlitz )

// RxJS v6+
import { interval } from 'rxjs';
import { mergeMap, take } from 'rxjs/operators';

// emit value every 1s
const source$ = interval(1000);

source$
  .pipe(
    mergeMap(
      // project
      val => interval(5000).pipe(take(2)),
      // resultSelector
      (oVal, iVal, oIndex, iIndex) => [oIndex, oVal, iIndex, iVal],
      // concurrent
      2
    )
  )
  /*
		Output:
		[0, 0, 0, 0] <--1st inner observable
		[1, 1, 0, 0] <--2nd inner observable
		[0, 0, 1, 1] <--1st inner observable
		[1, 1, 1, 1] <--2nd inner observable
		[2, 2, 0, 0] <--3rd inner observable
		[3, 3, 0, 0] <--4th inner observable
*/
  .subscribe(val => console.log(val));

Related Recipes

Additional Resources

📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/mergeMap.ts

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