Angular 2: Convert Observable to Promise
TypescriptAngularPromiseObservableTypescript Problem Overview
Q) How do I convert the following observable to a promise so I can call it with .then(...)
?
My method I want to convert to a promise:
this._APIService.getAssetTypes().subscribe(
assettypes => {
this._LocalStorageService.setAssetTypes(assettypes);
},
err => {
this._LogService.error(JSON.stringify(err))
},
() => {}
);
The service method it calls:
getAssetTypes() {
var method = "assettype";
var url = this.apiBaseUrl + method;
return this._http.get(url, {})
.map(res => <AssetType[]>res.json())
.map((assettypes) => {
assettypes.forEach((assettypes) => {
// do anything here you might need....
});
return assettypes;
});
}
Thanks!
Typescript Solutions
Solution 1 - Typescript
rxjs7
lastValueFrom(of('foo'));
https://indepth.dev/posts/1287/rxjs-heads-up-topromise-is-being-deprecated
rxjs6
https://github.com/ReactiveX/rxjs/issues/2868#issuecomment-360633707
> Don't pipe. It's on the Observable object by default. > > Observable.of('foo').toPromise(); // this
rxjs5
import 'rxjs/add/operator/toPromise';
import 'rxjs/add/operator/map';
...
this._APIService.getAssetTypes()
.map(assettypes => {
this._LocalStorageService.setAssetTypes(assettypes);
})
.toPromise()
.catch(err => {
this._LogService.error(JSON.stringify(err));
});
Solution 2 - Typescript
observable can be converted to promise like this:
let promise=observable.toPromise();
Solution 3 - Typescript
you dont really need to do this just do ...
import 'rxjs/add/operator/first';
this.esQueryService.getDocuments$.first().subscribe(() => {
event.enableButtonsCallback();
},
(err: any) => console.error(err)
);
this.getDocuments(query, false);
first() ensures the subscribe block is only called once (after which it will be as if you never subscribed), exactly the same as a promises then()
Solution 4 - Typescript
The proper way to make Observable a Promise, in your case would be following
getAssetTypesPromise() Observable<any> {
return new Promise((resolve, reject) => {
this.getAssetTypes().subscribe((response: any) => {
resolve(response);
}, reject);
});
}
Solution 5 - Typescript
Edit:
.toPromise()
is now deprecated in RxJS 7 (source: https://rxjs.dev/deprecations/to-promise)
New answer:
> As a replacement to the deprecated toPromise() method, you should use > one of the two built in static conversion functions firstValueFrom or > lastValueFrom.
Example:
import { interval, lastValueFrom } from 'rxjs';
import { take } from 'rxjs/operators';
async function execute() {
const source$ = interval(2000).pipe(take(10));
const finalNumber = await lastValueFrom(source$);
console.log(`The final number is ${finalNumber}`);
}
execute();
// Expected output:
// "The final number is 9"
Old answer:
A lot of comments are claiming toPromise
deprecated but as you can see here it's not.
So please juste use toPromise
(RxJs 6) as said:
//return basic observable
const sample = val => Rx.Observable.of(val).delay(5000);
//convert basic observable to promise
const example = sample('First Example')
.toPromise()
//output: 'First Example'
.then(result => {
console.log('From Promise:', result);
});
async/await example:
//return basic observable
const sample = val => Rx.Observable.of(val).delay(5000);
//convert basic observable to promise
const example = await sample('First Example').toPromise()
// output: 'First Example'
console.log('From Promise:', result);
Read more here.
Note: Otherwise you can use .pipe(take(1)).toPromise
but as said you shouldn't have any problem using above example.
Solution 6 - Typescript
toPromise is deprecated in RxJS 7.
Use:
lastValueFrom
Used when we are interested in the stream of values. Works like the former toPromise
Example
public async getAssetTypes() {
const assetTypes$ = this._APIService.getAssetTypes()
this.assetTypes = await lastValueFrom(assetTypes$);
}
firstValueFrom
Used when we are not interested in the stream of values but just the first value and then unsubscribe from the stream
public async getAssetTypes() {
const assetTypes$ = this._APIService.getAssetTypes()
this.assetTypes = await firstValueFrom(assetTypes$); // get first value and unsubscribe
}
Solution 7 - Typescript
You can convert Observable to promise just by single line of code as below:
let promisevar = observable.toPromise()
Now you can use then on the promisevar to apply then condition based on your requirement.
promisevar.then('Your condition/Logic');