Generics in Typescript - Undefined T

Question

Having a problem with Typescript's Generics where the type is undefined in the scope of the generic function or class. I can't find any documentation on this though I would assume it is by design. Is there a way to achieve what I am trying to, type-safely?

function test<T>() {
    return new T();
}

class TestClass<T> {
    public build(): T {
        return new T();
    }
}

Link to Play:

http://www.typescriptlang.org/Playground/#src=function%20test%3CT%3E()%20%7B%0A%09return%20new%20T()%3B%0A%7D%0A%0Aclass%20TestClass%3CT%3E%20%7B%0A%09public%20build()%3A%20T%20%7B%0A%09%09return%20new%20T()%3B%0A%09%7D%0A%7D%0A

For explanation of non-generic version, see stackoverflow.com/questions/13407036/… — Ryan Cavanaugh
– Ryan Cavanaugh, Commented Nov 13, 2015 at 4:36

basarat · Accepted Answer · 2015-11-13 04:33:37Z

3

TypeScript generics (unlike other languages like C#) are compile time only. So you cannot use them in runtime positions e.g. new T.

Is there a way to achieve what I am trying to, type-safely

Pass the constructor explicitly. e.g.

class TestClass<T> {
    public build(x:{new ():T}): T {
        return new x();
    }
}

Here x:{new ():T} I am saying that x is something that when called with new gives an instance of T.

answered Nov 13, 2015 at 4:33

basarat

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