what is new() in Typescript?

Question

I encountered new() in the official document here about generics.

Here is the code context:

function create(c: { new(): T; } )         


        

Answer 1

The new keyword is used to create an instance of a class, so in it's simplest form:

class SimpleClass {
}

Would be constructed as follows:

let simpleClassInstance = new SimpleClass();

This is all well and good, but how do you create an instance of a generic class, i.e:

class SimpleClassFactory< T > {
    static create( T ) {
        return new T(); // compile error could not find symbol T
    }
}

Would be used as follows:

let simpleClassInstance = SimpleClassFactory.create(SimpleClass);

Here, we are attempting to use the class definition to create an instance of a class. This will generate a compile error.

So we need to refer to the type by it's constructor signature:

class SimpleClassFactory< T > {
    static create( type: { new(): T ;} ) {
        return new type(); // succeeds
    }
}