Discovering the type of a value using TypeScript decorators

Take a look at my code snippet.

function observableDecorator<T>(target: T, key: keyof T) {
    let observable = ko.observable<any>((target[key] as any));

    Object.defineProperty(target, key, {
        get() {
            this[key]._ko_util_id = random;
            this.obs = observable<
                AT_THIS_POINT_THE_VALUE_TYPE_SHOULD_BE_INCLUDED
            >();
        },
        set(value) {
            observable(value);
        }
    });
}

I'm having trouble determining the type of value on line 6. I attempted to retrieve it from target[key], but it's returning undefined. Appreciate if someone can provide me with the correct solution. Will make sure to give credit to the helpful response.

Answer №1

Perhaps you will find this playground to be the solution you're seeking, as it shows potential for meeting your needs.

function observableDecorator<
    T extends Record<string, any>,
    K extends keyof T,
    V = T extends Record<string, infer X> ? X : never
>(target: T, key: K) {
    const observable = ko.observable<V>(target[key]);

    Object.defineProperty(target, key, {
        get() {
            this[key]._ko_util_id = random;
            this.obs = observable<V>(); // or typeof random?
        },
        set(value: V) {
            observable(value);
        }
    });
}

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