Typescript - How do I add an extension method

Question

I read that you can create extension methods in Typescript and I looked up some code

And put that code in my extension methods.ts but I get an error saying

Answer 1

You could extend the String interface, like this:

interface String {
    toNumber(): number;
}

String.prototype.toNumber = function(this: string) {
    return parseFloat(this);
}

const s = '123.45';
s.toNumber();

Answer 2

You can extend String interface by augmenting global scope:

export { };

declare global {
    interface String {
        toNumber(): number;
    }
}

String.prototype.toNumber = function (this: string) { return parseFloat(this) };

Playground