What is the type of a type in Typescript

Question

Here's a valid typescript snippet:

class A{
}

// what is the type of x ?  i.e. f( x: <TYPE HERE>)...
function f(x) {
    return new x();
}

const t = f(A);

It is obvious that the type of x is the type of the constructor function but it is not clear to me how one would specify it in Typescript.

Is it possible to type the parameter x ?

If yes, what is the type?

Answer 1

You can use:

interface Newable<T> {
    new (): T;
}

As follows:

class A {}

interface Newable<T> {
    new (): T;
}

function f(x: Newable<A>) {
    return new x();
}

const t = f(A);

If you want to allow constructor arguments you will need to type them:

class A {
    constructor(someArg1: string, someArg2: number) {
        // ...
    }
}

interface Newable<T> {
    new (someArg1: string, someArg2: number): T;
}

function f(x: Newable<A>) {
    return new x("someVal", 5);
}

const t = f(A);

And you could also do something more generic if needed:

interface Newable<T> {
    new (...args: any[]): T;
}