Import module into frontend app.js - nodejs, leaderline, javascript

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I think this may be easy, but I am new to it. I am making a web application using express and nodeJS where I want to use

npm install leader-line

in my frontend app.js. I can use the script on server/app.js but I will not be able to access the elements by using document.. The problem is, how do I import leader-line into my frontend app.js

This is my javascript at frontend.

function Start() {
     if(title === 'Tournament brackets')
        {
            let parent;
            let child;
            for (let base = 1; base < 5; base++) {
                let half = 1;
                for (let index = 1; index < 17; index++) {
                    parent = "parent" + index + "" + base;
                    if (index % 2 != 0) { 
                        while ( half <= Math.ceil(index / 2)){
                            child = "parent" + half + "" + (base + 1);
                            half++;
                        }
                    }
                    // Uncaught ReferenceError: LeaderLine is not defined
                    new LeaderLine(
                        document.getElementById(parent),
                        document.getElementById(child)
                    );
                }
            }
        }

For more references on LeaderLine.

Yes, I saw this but I am not sure how to implement it.

Thanks!

What I have tried:

I cannot use document. in app.js on server side.