Need suggestions on making jQuery fullstack app features modularized

This will be more of suggestion help rather than code problem help. Sorry this is very text heavy and broad topic.

Little background about the app:

We have an app that is written in Node.js, jQuery front end, ejs templates, and express for backend. It's a single page application app.

As of right now, we have multiple app that are built off one app. Each app has it own features but the core is the same.

We are currently facing a lot of problem with copy/paste feature from one app to another. Every time we implement new feature on one app, we just copy/paste the code from that app to other app.

Do you guys have any suggestions or open-source code that modularize features/code so that one can simply import/export those features into other app? Due to complexity of the app, we can't simply just import in a .js file because some feature requires adding a line of code to existing function/block of div, etc

For security purpose, I can't show example of the actual code. But the structure would be something like

drag-file.ejs (UI for dragging and drop file) drag-file.js (event-handler, any logic for drag and drop)