# mergeMap / flatMap

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

## 옵저버블과 매핑시킨 후 값을 발생시킵니다.

:bulb: flatMap과 mergeMap은 같은겁니다!

:bulb: 한번에 하나의 내부 subscription만 활성화해야하는 경우, [`switchMap`](/learn-rxjs-korean/learn-rxjs/recipes-1/transformation/switchmap.md)을 살펴보세요!

:bulb: 내부 옵저버블의 subscription과 값 발생의 순서가 중요하다면, [`concatMap`](/learn-rxjs-korean/learn-rxjs/recipes-1/transformation/concatmap.md)을 살펴보세요!

### 왜 `mergeMap` 을 사용할까요?

이 연산자는 내부 옵저버블은 평평하게 만들고 싶지만, 내부 subscription의 수는 조절하고싶을 때 가장 많이 쓰입니다.

예를 들어, when using [`switchMap`](/learn-rxjs-korean/learn-rxjs/recipes-1/transformation/switchmap.md) 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`](/learn-rxjs-korean/learn-rxjs/recipes-1/transformation/concatmap.md) 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`](/learn-rxjs-korean/learn-rxjs/recipes-1/filtering/take.md) or [`takeUntil`](/learn-rxjs-korean/learn-rxjs/recipes-1/filtering/takeuntil.md). You can also limit the number of active inner subscriptions at a time with the `concurrent` parameter, seen in [example 5](/learn-rxjs-korean/learn-rxjs/recipes-1/transformation/mergemap.md#example-5-mergemap-with-concurrent-value).

### Examples

**Example 1: mergeMap simulating save of click locations**

( [StackBlitz](https://stackblitz.com/edit/rxjs-xfwdnl?file=index.ts\&devtoolsheight=60) )

```javascript
// 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](https://stackblitz.com/edit/rxjs-wixf2a?file=index.ts\&devtoolsheight=60) )

```javascript
// 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](https://stackblitz.com/edit/typescript-pnnsrq?file=index.ts\&devtoolsheight=100) )

```javascript
// 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](https://stackblitz.com/edit/typescript-9p6ws7?file=index.ts\&devtoolsheight=100) )

```javascript
// 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](https://stackblitz.com/edit/typescript-r3gcr4?file=index.ts\&devtoolsheight=100) )

```javascript
// 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

* [Breakout Game](/learn-rxjs-korean/learn-rxjs/recipes/breakout-game.md)
* [HTTP Polling](/learn-rxjs-korean/learn-rxjs/recipes/http-polling.md)
* \[Save Indicator]\('../../recipes/save-indicator.md)
* [Swipe To Refresh](/learn-rxjs-korean/learn-rxjs/recipes/swipe-to-refresh.md)

### Additional Resources

* [mergeMap](https://rxjs.dev/api/operators/mergeMap) :newspaper: - Official

  docs
* [map vs flatMap](https://egghead.io/lessons/rxjs-rxjs-map-vs-flatmap)

  :video\_camera: :dollar: - Ben Lesh
* [Async requests and responses in RxJS](https://egghead.io/lessons/rxjs-04-reactive-programming-async-requests-and-responses-in-rxjs)

  :video\_camera: :dollar: - André Staltz
* [Use RxJS mergeMap to map and merge higher order observables](https://egghead.io/lessons/rxjs-use-rxjs-mergemap-to-map-and-merge-high-order-observables?course=use-higher-order-observables-in-rxjs-effectively)

  :video\_camera: :dollar: - André Staltz
* [Use RxJS mergeMap for fine grain custom behavior](https://egghead.io/lessons/rxjs-use-rxjs-mergemap-for-fine-grain-custom-behavior?course=use-higher-order-observables-in-rxjs-effectively)

  :video\_camera: :dollar: - André Staltz
* [Build your own mergeMap operator](https://blog.strongbrew.io/build-the-operators-from-rxjs-from-scratch/?lectureId=mergeMap#app)

  :video\_camera: - Kwinten Pisman

> :file\_folder: Source Code: <https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/mergeMap.ts>


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