How to assign a type that implements an interface?

Question

typescript

There is an interface

Interface MyInterface {
    myMethod(): string;
}

let MyInstance;

Next in the code, I want to indicate that myInstancethis is an instance of the class that implements MyInterface. How can I do that? If you do this:
let myInstance: MyInterface;then all unnecessary properties will give a compilation error, because an object literal is used, for example like this:

myInstance = {myMethod() {return ''}, a: 3} // property 'a' doesnt exist in MyInterface

This one, unfortunately, doesn't work either:
let myInstance: {} extends MyInterface;

Answer 1

MyInterface & { a: number }
But why?