Changing Observable to Promise in Angular 2

Question

Changing Observable to Promise in Angular 2

Q) What is the best way to convert an observable into a promise for easy handling with .then(...)?

The code snippet below showcases my current method that I am looking to transform into a promise:

  this._APIService.getAssetTypes().subscribe(
    assettypes => {
        this._LocalStorageService.setAssetTypes(assettypes);
    },
    err => {
        this._LogService.error(JSON.stringify(err))
    },
    () => {}
  );

Here is the service method being used in the above code:

  getAssetTypes() {
    var method = "assettype";
    var url = this.apiBaseUrl + method;

    return this._http.get(url, {})
      .map(res => <AssetType[]>res.json())
      .map((assettypes) => {
        assettypes.forEach((assettypes) => {
          // Perform any required actions here...
      });
      return assettypes;
    });      
  }

Your help on this matter is greatly appreciated. Thank you!

typescript angular promise observable

Answer 1

Answer №1

Latest usage of rxjs

lastValueFrom(of('foo'));

Find out more about the deprecation of topromise in rxjs

Newest release of rxjs

View comments on the latest updates for rxjs

Optimal use: It's included in the Observable object by default.
Observable.of('foo').toPromise(); // this

Recent version of rxjs

import 'rxjs/add/operator/toPromise';
import 'rxjs/add/operator/map';

...

this._APIService.getAssetTypes()
.map(assetTypes => {
  this._LocalStorageService.setAssetTypes(assetTypes);
})
.toPromise()
.catch(error => {
  this._LogService.error(JSON.stringify(error));
});

Answer 2

Latest usage of rxjs

lastValueFrom(of('foo'));

Find out more about the deprecation of topromise in rxjs

Newest release of rxjs

View comments on the latest updates for rxjs

Optimal use: It's included in the Observable object by default.
Observable.of('foo').toPromise(); // this

Recent version of rxjs

import 'rxjs/add/operator/toPromise';
import 'rxjs/add/operator/map';

...

this._APIService.getAssetTypes()
.map(assetTypes => {
  this._LocalStorageService.setAssetTypes(assetTypes);
})
.toPromise()
.catch(error => {
  this._LogService.error(JSON.stringify(error));
});

Answer 3

Answer №2

To convert an observable into a promise, you can use the following approach:

import { firstValueFrom, lastValueFrom } from 'rxjs';
...
lastValueFrom(observable).then(lastValue=>...);
firstValueFrom(observable).then(firstValue=>...);

The toPromise() method was the previous solution, but it has been deprecated starting from RxJS 7. The deprecated usage was:

let promise=observable.toPromise();

Answer 4

To convert an observable into a promise, you can use the following approach:

import { firstValueFrom, lastValueFrom } from 'rxjs';
...
lastValueFrom(observable).then(lastValue=>...);
firstValueFrom(observable).then(firstValue=>...);

The toPromise() method was the previous solution, but it has been deprecated starting from RxJS 7. The deprecated usage was:

let promise=observable.toPromise();

Answer 5

Answer №3

To convert Observable into a Promise in your specific scenario, the appropriate approach would be as follows:

convertObservableToPromise(): Promise<any> {
  return new Promise((resolve, reject) => {
      this.getAssetTypes().subscribe((response: any) => {
        resolve(response);
      }, reject);
    });
}

Answer 6

To convert Observable into a Promise in your specific scenario, the appropriate approach would be as follows:

convertObservableToPromise(): Promise<any> {
  return new Promise((resolve, reject) => {
      this.getAssetTypes().subscribe((response: any) => {
        resolve(response);
      }, reject);
    });
}

Answer 7

Answer №4

There is a more efficient way to handle this situation...

import 'rxjs/add/operator/first';

this.esQueryService.getDocuments$.first().subscribe(() => {
        event.enableButtonsCallback();
      },
      (err: any) => console.error(err)
    );
    this.getDocuments(query, false);

By using the first() method, you can ensure that the subscribe block is only executed once, similar to how a promise's then() method works.

Answer 8

There is a more efficient way to handle this situation...

import 'rxjs/add/operator/first';

this.esQueryService.getDocuments$.first().subscribe(() => {
        event.enableButtonsCallback();
      },
      (err: any) => console.error(err)
    );
    this.getDocuments(query, false);

By using the first() method, you can ensure that the subscribe block is only executed once, similar to how a promise's then() method works.

Answer 9

Answer №5

Update:

.toPromise() has been marked as deprecated in RxJS version 7 (source: https://rxjs.dev/deprecations/to-promise)

New solution:

To replace the deprecated toPromise() method, it is recommended to use either firstValueFrom or lastValueFrom, which are two built-in static conversion functions.

For 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"

Previous suggestion:

Despite some comments claiming that toPromise is deprecated, the method remains active according to this source.

Thus, you can still utilize toPromise in RxJs 6 as demonstrated below:

//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);

Further information available here.

Note: Alternatively, you may consider using .pipe(take(1)).toPromise, although the provided examples should suffice without any issues.

Answer 10

Update:

.toPromise() has been marked as deprecated in RxJS version 7 (source: https://rxjs.dev/deprecations/to-promise)

New solution:

To replace the deprecated toPromise() method, it is recommended to use either firstValueFrom or lastValueFrom, which are two built-in static conversion functions.

For 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"

Previous suggestion:

Despite some comments claiming that toPromise is deprecated, the method remains active according to this source.

Thus, you can still utilize toPromise in RxJs 6 as demonstrated below:

//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);

Further information available here.

Note: Alternatively, you may consider using .pipe(take(1)).toPromise, although the provided examples should suffice without any issues.

Answer 11

Answer №6

It's important to note that the toPromise method is deprecated in RxJS 7.

Instead, you can use:

lastValueFrom

This should be used when we want to access a continuous stream of values, similar to how toPromise functioned previously.

For example:

public async getAssetTypes() {
  const assetTypes$ = this._APIService.getAssetTypes()
  this.assetTypes = await lastValueFrom(assetTypes$);
}

firstValueFrom

On the other hand, if we are only interested in the first value from a stream and then want to unsubscribe, we can utilize this method.

public async getAssetTypes() {
  const assetTypes$ = this._APIService.getAssetTypes()
  this.assetTypes = await firstValueFrom(assetTypes$); // obtain first value and stop listening
}

Answer 12

It's important to note that the toPromise method is deprecated in RxJS 7.

Instead, you can use:

lastValueFrom

This should be used when we want to access a continuous stream of values, similar to how toPromise functioned previously.

For example:

public async getAssetTypes() {
  const assetTypes$ = this._APIService.getAssetTypes()
  this.assetTypes = await lastValueFrom(assetTypes$);
}

firstValueFrom

On the other hand, if we are only interested in the first value from a stream and then want to unsubscribe, we can utilize this method.

public async getAssetTypes() {
  const assetTypes$ = this._APIService.getAssetTypes()
  this.assetTypes = await firstValueFrom(assetTypes$); // obtain first value and stop listening
}

Answer 13

Answer №7

Converting an Observable to a promise is as simple as one line of code:

const myPromise = myObservable.toPromise();

You can then use the `then` method on the `myPromise` variable to implement your specific conditions or logic.

myPromise.then('Implement your condition/logic here');

Answer 14

Converting an Observable to a promise is as simple as one line of code:

const myPromise = myObservable.toPromise();

You can then use the `then` method on the `myPromise` variable to implement your specific conditions or logic.

myPromise.then('Implement your condition/logic here');

Answer 15

Answer №8

My preference is for a raw approach now that toPromise() is no longer available

   const status = await new Promise<boolean>((resolve, reject) => {
     someObs$.subscribe({
      next: resolve,
      error: reject,
    });
  });

An elegant solution involves utilizing https://rxjs.dev/api/index/function/lastValueFrom

  const replyTo = new AsyncSubject();

  replyTo.next(false);
  replyTo.next(false);
  replyTo.next(true);

  replyTo.complete();

  const status = await lastValueFrom(replyTo) // true

Answer 16

My preference is for a raw approach now that toPromise() is no longer available

   const status = await new Promise<boolean>((resolve, reject) => {
     someObs$.subscribe({
      next: resolve,
      error: reject,
    });
  });

An elegant solution involves utilizing https://rxjs.dev/api/index/function/lastValueFrom

  const replyTo = new AsyncSubject();

  replyTo.next(false);
  replyTo.next(false);
  replyTo.next(true);

  replyTo.complete();

  const status = await lastValueFrom(replyTo) // true

Answer 17

Answer №9

Many have suggested using myObservable.toPromise(), but this functionality will be removed in a future version of rxJs. If you prefer to continue using toPromise() to avoid making changes across multiple projects, you can create your own 'extension method' that won't be deprecated. You can implement this solution now, even before toPromise() is officially removed from rxJs.

declare module "rxjs" {
    interface Observable<T> {
        /**
         * Extension method. Implements 'lastValueFrom' for Observable<T>.
         */
        toPromise(): Promise<T | undefined>;
    }
}

Observable.prototype.toPromise = function <T>(this: Observable<T>): Promise<T> {
    return lastValueFrom(this);
};

To ensure this modification is applied globally, it should be included in the file that initializes your application. In the case of Angular apps, this would typically be in main.js.

Answer 18

Many have suggested using myObservable.toPromise(), but this functionality will be removed in a future version of rxJs. If you prefer to continue using toPromise() to avoid making changes across multiple projects, you can create your own 'extension method' that won't be deprecated. You can implement this solution now, even before toPromise() is officially removed from rxJs.

declare module "rxjs" {
    interface Observable<T> {
        /**
         * Extension method. Implements 'lastValueFrom' for Observable<T>.
         */
        toPromise(): Promise<T | undefined>;
    }
}

Observable.prototype.toPromise = function <T>(this: Observable<T>): Promise<T> {
    return lastValueFrom(this);
};

To ensure this modification is applied globally, it should be included in the file that initializes your application. In the case of Angular apps, this would typically be in main.js.

Changing Observable to Promise in Angular 2

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Answer №6

Answer №7

Answer №8

Answer №9

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