Mutating signal from toSignal()

[rose tulip]

Hey, I have 2 questions regarding the new signals. I'm trying to simulate a slow call to an API which returns an observable.
Is it possible to get a WritableSignal<T> from the toSignal() function, instead of a signal<T>?
I want to fetch a user object and then mutate said object.

Also, on line 27, should typescript not infer that this.user() is not undefined with the type guard on line 23?

I know in this example I can pass inn an initial value to user but in reality this probably would not be feasible.

[surreal nymph]

toSignal returns Signal<T|undefined>

because signals have a default value

until the observable sends a value, signal is undefined

also that signal isn't supposed to be writable because it is "populated" by the observable

[lime pewter]

The docs suggest to provide an initialValue to toSignal to avoid this, but I'm wondering you could use a type predicate instead to check that you're actually dealing with a signal
https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates

[cyan forum]

Also, on line 27, should typescript not infer that this.user() is not undefined with the type guard on line 23?
No, because TypeScript can't know that the second function call returns the same value as the first one. Store the result of the first call in a variable, and use that variable.

[rose tulip]

Thanks for your answer, when I think about it this makes a lot of sense 👍