typescript - How to constraint a generic type to have new()?

Question

I want to have a function like:

createEntity<TEntity>(): TEntity {
    return new TEntity();
}

In C#, we could write:

void TEntity CreateEntity<TEntity>() where TEntity : new()

How can I do this in TypeScript?

Answer 1

The only way shown in the handbook to do something similar to this is to send the class you want to initialize as a parameter to the factory method, and describe it's constructor using the new keyword.

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

class SomeClass { }

let result = factory(SomeClass);

The result will be of type SomeClass.

The constructor of the class will be type checked against the interface defined in the factory method.

If you want to initialize a class that takes a parameter in it's constructor you will have to specify that in the interface given to the factory method.

function factory<T>(type: { new (...args): T }, ...args): T {
    return new type(...args);
}


class SomeClass {
    constructor(name: string) { }
}

let a = factory(SomeClass, 'John Doe');