Adding a function to a constructor from an external file

[DriedSponge]

How can I add a function onto this from an external file?
For example:

class SpongeHelpers{
    public html: any;
    constructor(sel) {
        if (!sel) return;
        if (typeof sel === 'function') {
            document.addEventListener('DOMContentLoaded', sel);
        }else if (typeof sel === 'string') {
            var selected = document.querySelectorAll(sel);
            console.log(this);
            this.html = (arg) => {
                selected.forEach((element) => {
                    element.innerHTML = arg;
                })
            };
        }

    }
    init(sel){
        return new SpongeHelpers(sel);
    }
}

As you can see, I define this.html within the class but I want to be able to define from an external file, so it's easier to add more functions in the future. I did some looking around and found out I need to do something with interfaces, however I do not know how to properly use them. Any help is much appreciated.

[DriedSponge]

Solution: https://stackoverflow.com/questions/50155811/add-function-to-class-from-another-file-in-typescript