Fiddler
I think you know what happened, by now.
I was watching a video of a guy making this JavaScript fiddle thingy, and I got caught up, and so I ended up using mare-dom to make something of the sort. You can also check it out in its own repo, where it will probably be updated, and on GitHub pages.
It's honestly one of the most awesome things I have ever made, mostly because I made it from scratch, using a library I built myself for frontend.
I hope you find this entertaining! Basically, what this does, is it evaluetes inline JavaScript functions, and it outputs the result of the computations.
You can add and delete segments through a commander, which you can launch by pressing ctrl+alt+P, by either clicking on the icons or pressing the relative keyboard shortcuts.
You can then press CTRL+ENTER to execute a fiddle!
Have fun!