# RXJS

WalletAPIServer uses a bit of rxjs (version 7) to handle data coming in from .\
You should be familiar with it (if not, start here [https://www.learnrxjs.io/#brand-new-to-rxjs](https://www.learnrxjs.io/#brand-new-to-rxjs)).
In case you just need a refresher, you'll find below all the different rxjs operators and functions used in wallet-api-server.

## map

[https://rxjs.dev/api/operators/map](https://rxjs.dev/api/operators/map)

## observable

[https://rxjs.dev/guide/observable](https://rxjs.dev/guide/observable)

## BehaviorSubject

[https://www.learnrxjs.io/learn-rxjs/subjects/behaviorsubject](https://www.learnrxjs.io/learn-rxjs/subjects/behaviorsubject)

> Requires an initial value and emits the current value to new subscribers

```js
// RxJS v6+
import { BehaviorSubject } from "rxjs";

const subject = new BehaviorSubject(123);

// two new subscribers will get initial value => output: 123, 123
subject.subscribe(console.log);
subject.subscribe(console.log);

// two subscribers will get new value => output: 456, 456
subject.next(456);

// new subscriber will get latest value (456) => output: 456
subject.subscribe(console.log);

// all three subscribers will get new value => output: 789, 789, 789
subject.next(789);

// output: 123, 123, 456, 456, 456, 789, 789, 789
```

## combineLatest

[https://www.learnrxjs.io/learn-rxjs/operators/combination/combinelatest](https://www.learnrxjs.io/learn-rxjs/operators/combination/combinelatest)

> When any observable emits a value, emit the last emitted value from each.

let's look at this function

```js
const allowedCurrencies$ = new BehaviorSubject([]);

combineLatest(
  [this.allCurrencies$, this.permissions.currencyIds$],
  matchCurrencies
).subscribe(allowedCurrencies$);
```

Here, allowedCurrencies emit first an empty array, then it emits a new value:
anytime, `this.allCurrencies$` or `this.permissions.currencyIds$` changes
Those two values are passed to a projection function (which simply finds the currencies we have defined [here](../../core/modules/currency))

[![](https://markdown-videos-api.jorgenkh.no/youtube/jI8GSJgPrpI)](https://youtu.be/jI8GSJgPrpI)

## firstValueFrom

[https://rxjs.dev/api/index/function/firstValueFrom](https://rxjs.dev/api/index/function/firstValueFrom)

> Converts an observable to a promise by subscribing to the observable, and returning a promise that will resolve as soon as the first value arrives from the observable. The subscription will then be closed.

[![](https://markdown-videos-api.jorgenkh.no/youtube/Pqfc7xa0QTg)](https://youtu.be/Pqfc7xa0QTg)