Quick Fix: In Angular HTTP, lastValueFrom and firstValueFrom are equivalent because observer.complete() is called synchronously after observer.next(). The only difference is that firstValueFrom resolves the Promise on next(), while lastValueFrom resolves it on complete(). Use lastValueFrom with reportProgress: true to catch the last emitted value (response or error), not progress updates.
The Problem:
In Angular, the HttpClient service provides a method called get() that returns an observable. To convert this observable into a promise, we can use the lastValueFrom() or firstValueFrom() operators. The main question is: in cases where the observable emits multiple values, will the results of using lastValueFrom() and firstValueFrom() be the same?
The Solutions:
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Solution 1: lastValueFrom and firstValueFrom equivalence in Angular HTTP
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In Angular HTTP client, lastValueFrom and firstValueFrom are equivalent, as they both resolve the promise once the HTTP request is complete. The difference between them lies in when the promise is resolved: firstValueFrom resolves the promise after the first value is emitted, while lastValueFrom resolves it after the last value is emitted. Since the HTTP client emits only one value and then completes the observable, there is no practical difference between using lastValueFrom and firstValueFrom in this context.
However, if you enable progress reporting (reportProgress: true), lastValueFrom is the better choice as it will capture the last emitted value, which is either the response or an error. This approach ensures that you do not resolve the Promise with a progress status update, which is undesirable.
Solution 2: Use any one as per your choice
While performing the GET requests using Angular HttpClient, if you don’t use additional options like observe: events or reportProgress: true, both lastValueFrom and firstValueFrom will yield the same result. This is because the observable will emit only once and in this case, either of these solutions would work and provide the desired result.
Therefore, the choice of using lastValueFrom or firstValueFrom in this context is primarily a matter of preference. Both are equivalent and will serve the purpose of retrieving the single emitted value from the observable. Ultimately, the decision of which one to use may depend on personal preference or specific project requirements.
Solution 3: lastValueFrom and firstValueFrom Equivalence
lastValueFrom and firstValueFrom are two operators in Angular HTTP that allow you to convert an observable into a promise. They are similar in that they both return the last or first value emitted by the observable, respectively. However, there are some key differences between the two operators:
- Completeness: lastValueFrom requires the observable to complete before it can return a value, while firstValueFrom does not. This means that lastValueFrom can be used with observables that never complete, such as interval() or timer(), while firstValueFrom cannot.
- Emission: lastValueFrom returns the last value emitted by the observable, while firstValueFrom returns the first value emitted by the observable.
- Errors: lastValueFrom will throw an error if the observable emits an error, while firstValueFrom will ignore errors and return undefined.
To illustrate the differences between the two operators, consider the following example:
const observable$ = interval(1000);
const lastValue$ = lastValueFrom(observable$);
const firstValue$ = firstValueFrom(observable$);
lastValue$.then((value) => {
console.log(`Last value: ${value}`);
});
firstValue$.then((value) => {
console.log(`First value: ${value}`);
});
In this example, the observable$ is an interval observable that emits a value every second. The lastValue$ observable will wait until the observable$ completes (which it never will) before returning a value. The firstValue$ observable, on the other hand, will return the first value emitted by the observable$, which will be the number 0.
Conclusion:
lastValueFrom and firstValueFrom are two useful operators for converting observables into promises. However, they have different behaviors and should be used in different situations. lastValueFrom should be used when you need to wait for the observable to complete before getting a value, while firstValueFrom should be used when you only need the first value emitted by the observable.
