Object extension function TypeScript Angular 6

Question

why is when I attempt the code

declare interface String {
  testsdfsdffds();
}

String.prototype.testsdfsdffds = () => {

};

it works, but substituting Object for String causes errors. I thought you could extend Object without issues?

My goal really is to mimic the Kotlin apply and let extension functions - but I would like to understand the how/why you can/cant extend objects?