> For the complete documentation index, see [llms.txt](https://www.learnrxjs.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://www.learnrxjs.io/learn-rxjs/operators/transformation/concatmapto.md).

# concatMapTo

#### signature: `concatMapTo(observable: Observable, resultSelector: function): Observable`

## Subscribe to provided observable when previous completes, emit values.

### Examples

**Example 1: Map to basic observable (simulating request)**

( [StackBlitz](https://stackblitz.com/edit/typescript-fkkh6c?file=index.ts\&devtoolsheight=50) )

```js
// RxJS v6+
import { of, interval } from 'rxjs';
import { concatMapTo, delay, take } from 'rxjs/operators';

//emit value every 2 seconds
const sampleInterval = interval(500).pipe(take(5));
const fakeRequest = of('Network request complete').pipe(delay(3000));
//wait for first to complete before next is subscribed
const example = sampleInterval.pipe(concatMapTo(fakeRequest));
//result
//output: Network request complete...3s...Network request complete'
const subscribe = example.subscribe(val => console.log(val));
```

**Example 2: Using projection with `concatMap`**

( [StackBlitz](https://stackblitz.com/edit/typescript-8kcfm1?file=index.ts\&devtoolsheight=100) | [jsBin](http://jsbin.com/fogefebisu/1/edit?js,console) | [jsFiddle](https://jsfiddle.net/btroncone/s19wtscb/) )

```js
// RxJS v6+
import { interval } from 'rxjs';
import { concatMapTo, take } from 'rxjs/operators';
//emit value every 2 seconds
const interval$ = interval(2000);
//emit value every second for 5 seconds
const source = interval(1000).pipe(take(5));
/*
  ***Be Careful***: In situations like this where the source emits at a faster pace
  than the inner observable completes, memory issues can arise.
  (interval emits every 1 second, basicTimer completes every 5)
*/
// basicTimer will complete after 5 seconds, emitting 0,1,2,3,4
const example = interval$.pipe(
  concatMapTo(
    source,
    (firstInterval, secondInterval) => `${firstInterval} ${secondInterval}`
  )
);
/*
  output: 0 0
          0 1
          0 2
          0 3
          0 4
          1 0
          1 1
          continued...

*/
const subscribe = example.subscribe(val => console.log(val));
```

### Additional Resources

* [concatMapTo](https://rxjs.dev/api/operators/concatMapTo) 📰 - Official docs

***

> 📁 Source Code: <https://github.com/ReactiveX/rxjs/blob/master/packages/rxjs/src/internal/operators/concatMapTo.ts>


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://www.learnrxjs.io/learn-rxjs/operators/transformation/concatmapto.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.