Create a new object from type parameter in generic class

To create a new object within generic code, you need to refer to the type by its constructor function. So instead of writing this:

function activatorNotWorking<T extends IActivatable>(type: T): T {
    return new T(); // compile error could not find symbol T
}

You need to write this:

function activator<T extends IActivatable>(type: { new(): T ;} ): T {
    return new type();
}

var classA: ClassA = activator(ClassA);

See this question: Generic Type Inference with Class Argument

Because the compiled JavaScript has all the type information erased, you can't use T to new up an object.

You can do this in a non-generic way by passing the type into the constructor.

class TestOne {
    hi() {
        alert('Hi');
    }
}

class TestTwo {
    constructor(private testType) {

    }
    getNew() {
        return new this.testType();
    }
}

var test = new TestTwo(TestOne);

var example = test.getNew();
example.hi();

You could extend this example using generics to tighten up the types:

class TestBase {
    hi() {
        alert('Hi from base');
    }
}

class TestSub extends TestBase {
    hi() {
        alert('Hi from sub');
    }
}

class TestTwo<T extends TestBase> {
    constructor(private testType: new () => T) {
    }

    getNew() : T {
        return new this.testType();
    }
}

//var test = new TestTwo<TestBase>(TestBase);
var test = new TestTwo<TestSub>(TestSub);

var example = test.getNew();
example.hi();

I know late but @TadasPa's answer can be adjusted a little by using

TCreator: new() => T

instead of

TCreator: { new (): T; }

so the result should look like this

class A {
}

class B<T> {
    Prop: T;
    constructor(TCreator: new() => T) {
        this.Prop = new TCreator();
    }
}

var test = new B<A>(A);

All type information is erased in JavaScript side and therefore you can't new up T just like @Sohnee states, but I would prefer having typed parameter passed in to constructor:

class A {
}

class B<T> {
    Prop: T;
    constructor(TCreator: { new (): T; }) {
        this.Prop = new TCreator();
    }
}

var test = new B<A>(A);