Load javascript as an ES6 module OR via a script tag in the page

Fairly new to JS modules of any kind. I have a simple vanilla JS script that is intended to be loaded into an html document via the script tag. Basically...

// invoke main function when DOM is ready
document.addEventListener("DOMContentLoaded", function() {
    MainFunction();
});

var MainFunction = function() {
    // check for specific DOM nodes upon invocation...
}

Works great. But I'd like to also be able to make MainFunction available as a module export so I can import it into a webpack bundle like:

import MainFunction from 'myclassicscript';

And then I can invoke it as necessary in the main bundle with:

MainFunction();

How the same script can be used both ways. I can't seem to find any info on how to do it, but I might have totally the wrong idea.

Something like...?

// check if this is a browser environment...
if (typeof window !== 'undefined') {

    // invoke main function when DOM is ready
    document.addEventListener("DOMContentLoaded", function() {
        MainFunction();
    });

}

var MainFunction = function() {
    // check for specific DOM nodes upon invocation...
}

// ???
export default MainFunction;