Typing constructor functions in TypeScript

I'm having problems understanding how to correctly type a constructor function in TS. I'm aware that a common answer would be to use a class, but I'm currently exploring TS features. In my tsconfig.json I have noImplicitAny: true.

My constructor function looks like this in JS:

function User(username) {
     this.username = username
}
User.prototype.age = 57

My first attempt to type the functions relied on the TS docs that explain how to define call signatures:

interface IUser {
    username: string;
    age: number;
}

type UserConstructor = {
    (username: string): void;
    new (username: string): IUser;
}

const User: UserConstructor = function(username: string) {
    this.username = username
}
User.prototype.age = 57

The compiler complains since the value of this is unknown and the call signatures don't seem to match.

After doing some research, I was able to come up with a version that works but I don't really like it:

interface ICUser {
    username: string;
    age: number;
}

function ConstructedUser(this:ICUser, username: string) {
    this.username = username;
}

ConstructedUser.prototype.dateOfBirth = "Freb"

Here's what I don't like about solution nr. 2:

• I'm not sure I completely understand the explicit passing of this since there aren't any official docs on this.
• The typing is very verbose and does not separate between properties on the instance and prototype chain.

I wasn't able to find good resources on how to type constructor functions and hoped someone might be able to help.