Typescript prototype extension is not a function

Currently I am trying to write a prototype extension function. But it seems that nothing what I try works. Have for that reason also tried examples from others but they all give the same error: TypeError: array.remove is not a function.

Here is some code I put in my generic-utill file:

export {};
declare global {
    interface Array<T> {
        remove(elem: T): Array<T>;
    }
}

Array.prototype.remove = function<T>(elem: T): T[] {
    return this.filter(e => e !== elem);
};

And this in my service class:

const array = [1, 2, 3, 4];
const result = array.remove(1);

Could someone maybe explain what I am doing wrong since I don’t really understand what I am missing.