Is it possible to determine the type of a class-type instance using class decorators?

Question

Is it possible to determine the type of a class-type instance using class decorators?

function DecorateClass<T>(instantiate: (...params:any[]) => T){
    return (classTarget:T) => { /*...*/ }
}

@DecorateClass((json:any) => {
    //This is just an example, the key is to ensure it returns
    //an instance specific to the class being decorated.
    var instance = new Animal();
    instance.Name = json.name;
    instance.Sound = json.sound;
    return instance;
})
class Animal {
    public Name:string;
    public Sound:string;
}

I aim to enforce that the anonymous function within the decorator always produces an instance of the relevant class. However, the existing code fails because T actually refers to typeof Animal and not just Animal.

In a generic function, is there any way to derive type Animal from typeof Animal without needing overly detailed definitions like

function DecorateClass<TTypeOfClass, TClass>

?

It's unfortunate that utilizing typeof in the generic syntax isn't allowed, which was my initial approach to align with what I intend:

function DecorateClass<T>(instantiate: (json:any) => T){
    return (classTarget:typeof T) => { /*...*/  } // Cannot resolve symbol T
}

typescript

Answer 1

Answer №1

Wait a moment...

I recently encountered the need for a function type definition that takes a class as an argument and returns an instance of that class. After coming up with a solution, it prompted me to ponder on this question.

By using a newable type, a relationship between a class and its instance can be established, providing a precise solution to your query:

function DecorateClass<T>(instantiate: (...args: any[]) => T) {
    return (classTarget: { new(...args: any[]): T }) => { /*...*/ }
}

Explanation

In TypeScript, any newable type can be defined with the signature:

new(...args: any[]): any

This represents a constructor function that may or may not take arguments and returns an instance, which could also be a generic type.

By inferring the return type from the constructor function, we can define the return type of the callback function passed into the decorator based on the class it is applied to.

Upon testing the decorator, it functions exactly as intended:

@DecorateClass((json: any) => {
    return new Animal(); // OK
})
@DecorateClass((json: any) => {
    return Animal; // Error
})
@DecorateClass((json: any) => {
    return "animal"; // Error
})
class Animal {
    public Name: string;
    public Sound: string;
}

This essentially overrides my previous response.

Edit: Inheritance

When dealing with inheritance (e.g., returning a derived type from instantiate), assignability behaves differently: you can return a base type but not a derived type.

This discrepancy occurs because the returned type from instantiate supersedes the "returned" type of classTarget in generic type inference. For further insight into this issue, refer to the following question:

Generic type parameter inference priority in TypeScript

Answer 2

Wait a moment...

I recently encountered the need for a function type definition that takes a class as an argument and returns an instance of that class. After coming up with a solution, it prompted me to ponder on this question.

By using a newable type, a relationship between a class and its instance can be established, providing a precise solution to your query:

function DecorateClass<T>(instantiate: (...args: any[]) => T) {
    return (classTarget: { new(...args: any[]): T }) => { /*...*/ }
}

Explanation

In TypeScript, any newable type can be defined with the signature:

new(...args: any[]): any

This represents a constructor function that may or may not take arguments and returns an instance, which could also be a generic type.

By inferring the return type from the constructor function, we can define the return type of the callback function passed into the decorator based on the class it is applied to.

Upon testing the decorator, it functions exactly as intended:

@DecorateClass((json: any) => {
    return new Animal(); // OK
})
@DecorateClass((json: any) => {
    return Animal; // Error
})
@DecorateClass((json: any) => {
    return "animal"; // Error
})
class Animal {
    public Name: string;
    public Sound: string;
}

This essentially overrides my previous response.

Edit: Inheritance

When dealing with inheritance (e.g., returning a derived type from instantiate), assignability behaves differently: you can return a base type but not a derived type.

This discrepancy occurs because the returned type from instantiate supersedes the "returned" type of classTarget in generic type inference. For further insight into this issue, refer to the following question:

Generic type parameter inference priority in TypeScript

Answer 3

Answer №2

Modify

It appears that the request you are making is entirely feasible. I have introduced a fresh response, but opted to keep this one intact since it could contain valuable insights for someone else. While this answer proposes a runtime approach, the new one suggests a solution at compile time.

In my opinion, implementing runtime type checking would be your best option, as you will certainly have the accurate type within the decorator function:

function DecorateClass(instantiate: (...params: any[]) => any) {
    return (classTarget: Function) => {
        var instance = instantiate(/*...*/);

        if (!(instance instanceof classTarget)) {
            throw new TypeError();
        }

        // ...
    }
}

However, please note that this approach does not guarantee compile-time type safety.

Answer 4

Modify

It appears that the request you are making is entirely feasible. I have introduced a fresh response, but opted to keep this one intact since it could contain valuable insights for someone else. While this answer proposes a runtime approach, the new one suggests a solution at compile time.

In my opinion, implementing runtime type checking would be your best option, as you will certainly have the accurate type within the decorator function:

function DecorateClass(instantiate: (...params: any[]) => any) {
    return (classTarget: Function) => {
        var instance = instantiate(/*...*/);

        if (!(instance instanceof classTarget)) {
            throw new TypeError();
        }

        // ...
    }
}

However, please note that this approach does not guarantee compile-time type safety.

Is it possible to determine the type of a class-type instance using class decorators?

Answer №1

Explanation

Edit: Inheritance

Answer №2

Modify

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