Paleo Paleo - 1 month ago 19
TypeScript Question

Generics in TypeScript: How to infer the type of an instance from the class

A factory function creates the instances of classes:

class A {
name: string
}
function factory<T>(Cl): T {
return new Cl()
}
let a = factory<A>(A)
a.name // OK


I would like to avoid the repetition of
A
in:
factory<A>(A)
. The generics instance type should be able to be inferred from the class type, shouldn't be?

I tried this code:

function factory<T>(Cl: typeof T): T { // Error: Cannot find name 'T'
return new Cl()
}


Is there a way to do this?

Answer

Based on the Typescript documentation :

When creating factories in TypeScript using generics, it is necessary to refer to class types by their constructor functions.

So you must do something like this:

function factory<T>(Cl: { new(): T; }): T {
    return new Cl();
}

In the code above, Cl must a type that at least has a constructor which return T generic type.

So the type inference will work:

let a = factory(A);
a.name;

You don't need to specify the type of A anyway because the compiler know it.

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