Using typescript 3.7, I have an interface with a property that is meant to accept a constructor function:
interface IConstruct<T> {
type: new (...args:ConstructorParameters<T>) => T;
}
My thought is that IConstruct<User> will have a property {type: User}.
But the compiler tells me that T cannot be used there. Why is that?
TS2344: Type T does not satisfy the constraint 'new (...args: any) => any'
The type of ConstructorParameters looks like this:
type ConstructorParameters<T extends new (...args: any) => any> =
T extends new (...args: infer P) => any ? P : never;
So type parameter T itself has to extend some sort of constructor function defined by the constraint extends new (...args: any) => any. Write above example like this and you should be good to go:
class User {
constructor(public name: string) { }
}
// add constructor function type constraint for T
interface IConstruct<T extends new (...args: any) => any> {
// we can use built-in InstanceType to infer instance type from class type
type: new (...args: ConstructorParameters<T>) => InstanceType<T>;
}
type UserConstruct = IConstruct<typeof User>
const constr: UserConstruct = {
type: User
}
constr.type // new (name: string) => User
const userInstance = new constr.type("John") // userInstance: User
console.log(userInstance.name) // John
T1 you used typeof User instead of just User ? Isn't User already a type?
T when invoking IConstruct.type constructor function (that is probably what you want here). I also gave a bit more details to the example, hope it helps.
ConstructorParameters, what is ? P : never? Is never a reference to a constructor that is never executed - i.e., the constructor signature of an abstract class?? : construct is a conditional type, never the bottom type in the type system. That means nothing is assignable to it, so you can use it here as some kind of type error indication, if TS is not able to infer the constructor parameters.The Problem has been resolved: https://github.com/microsoft/TypeScript/issues/31278
For me the missing typeof was the problem:
class Parent<T> {
constructor(readonly data: T) {
}
}
class Child<T> extends Parent<T> {
constructor(...args: ConstructorParameters<typeof Parent<T>>) {
super(...args);
}
}
