Type information is lost when retrieving Subscribable<T> value

[craxal]

Consider the following code:

import * as ko from "knockout";
let obj: ko.Subscribable<number> = ko.observable(5);
let result = obj();

Here, I expect TypeScript to deduce the type for result to be number. However, it deduces the type to be any. This can be attributed to the fact that the base type (SubscribableFunctions<T>) extends Function, which discards the type parameter.

This can be fixed by modifying the Subscribable<T> type declaration to reflect Observable<T>:

export interface Subscribable<T = any> extends SubscribableFunctions<T> {
    (): T
}

[sumitkm]

You have to define the type of the observable as follows

import * as ko from "knockout";
let obj = ko.observable<number>(5);
let result = obj();

Then TypeScript will infer type of result as number

[craxal]

@sumitkm Did a little more digging, and I've updated the example to more closely resemble the situation we're running into. There is no problem with Observable<T> type, but that's not the type I'm concerned about. There is a problem with Subscribable<T>. This is a type we use, because there are cases where we want to use an observable or a computed, and Subscribable<T> is their common base type.