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merge

signature: merge(input: Observable): Observable

Turn multiple observables into a single observable.

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💡 This operator can be used as either a static or instance method!
💡 If order not throughput is a primary concern, try concat instead!
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Why use merge?

The merge operator is your go-to solution when you have multiple observables that produce values independently and you want to combine their output into a single stream. Think of it as a highway merger, where multiple roads join together to form a single, unified road - the traffic (data) from each road (observable) flows seamlessly together.
A real-world example can be seen in a chat application, where you have separate observables for receiving messages from multiple users. By using merge, you can bring all those message streams into a single unified stream for displaying the messages in the chat window.
Keep in mind that merge will emit values as soon as any of the observables emit a value. This is different from combineLatest or withLatestFrom, which wait for each observable to emit at least one value before emitting a combined value.
Lastly, if you're dealing with observables that emit values at specific intervals and you need to combine them based on time, consider using the zip operator instead.
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Examples

Example 1: merging multiple observables, static method
// RxJS v6+
import { mapTo } from 'rxjs/operators';
import { interval, merge } from 'rxjs';
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//emit every 2.5 seconds
const first = interval(2500);
//emit every 2 seconds
const second = interval(2000);
//emit every 1.5 seconds
const third = interval(1500);
//emit every 1 second
const fourth = interval(1000);
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//emit outputs from one observable
const example = merge(
first.pipe(mapTo('FIRST!')),
second.pipe(mapTo('SECOND!')),
third.pipe(mapTo('THIRD')),
fourth.pipe(mapTo('FOURTH'))
);
//output: "FOURTH", "THIRD", "SECOND!", "FOURTH", "FIRST!", "THIRD", "FOURTH"
const subscribe = example.subscribe(val => console.log(val));
Example 2: merge 2 observables, instance method
// RxJS v6+
import { merge } from 'rxjs/operators';
import { interval } from 'rxjs';
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//emit every 2.5 seconds
const first = interval(2500);
//emit every 1 second
const second = interval(1000);
//used as instance method
const example = first.pipe(merge(second));
//output: 0,1,0,2....
const subscribe = example.subscribe(val => console.log(val));

Additional Resources

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Last modified 1mo ago