NestJs async httpService call

rxjs library is most powerful concurrency package that chosen form handling system event like click, external request like get data or delete record and ....

The main concept behind this library is:

handle data that receive in future

therefor you most use 3 argument in observable object like

observablSource.subscribe(
   data => { ... },
   failure => { ... },
   compelete => { ... }
)

but for most backend developer use Promises that comes from ECMAScript 6 feature and is native part of JavaScript.

By default in Angular 4+ and Nest.js use rxjs that support Observable. In technical details you can find a solution for change automatic observable to promise.

const data: Observable<any>;
data.from([
   {
      id: 1,
      name: 'mahdi'
   }, 
   {
      id: 2,
      name: 'reza'
   },
 ])

now you have simulate a request with observable type from server. if you want to convert it to Pormise use chained method like example:

   data.toPromise();

from this step you have promised object and form using it better to attach async/await

 async userList( URL: string | URLPattern ) {
    const userList = await this.http.get<any>( URL ).toPromise();
    ...
 }

As toPromise() is being deprecated, you can replace it with firstValueFrom or lastValueFrom

For example:

const resp = await firstValueFrom(this.http.post(`http://localhost:3000/myApi`)

https://rxjs.dev/deprecations/to-promise

try these below one instead of only async-await.

There are some basic reasons behind the deprecation of toPromise.

We know that Promise and Observables that both collections may produce values over time. where Observables return none or more and Promise return only one value when resolve successfully.

there is a main reason behind the seen

• Official: https://rxjs.dev/deprecations/to-promise
• Stack:Rxjs toPromise() deprecated

Now we have 2 new functions.

• lastValueFrom : last value that has arrived when the Observable completes for more https://rxjs.dev/api/index/function/firstValueFrom

• firstValueFrom: you might want to take the first value as it arrives without waiting an Observable to complete for more https://rxjs.dev/api/index/function/lastValueFrom

The HttpModule uses Observable not Promise which doesn't work with async/await. All HttpService methods return Observable<AxiosResponse<T>>.

So you can either transform it to a Promise and then use await when calling it or just return the Observable and let the caller handle it.

create(data): Promise<AxiosResponse> {
    return this.httpService.post(url, data).toPromise();
                                           ^^^^^^^^^^^^^
}

Note that return await is almost (with the exception of try catch) always redundant.

Update 2022

toPromise is deprecated. Instead, you can use firstValueFrom:

import { firstValueFrom } from 'rxjs';

// ...

return firstValueFrom(this.httpService.post(url, data))