Modernize our Javascript

[yuvipanda]

Our JS could use some love and modernizing - to make it easier to reason about, catch mistakes (like #1375), and modify (rather than just for the sake of modernizing). This isn't about using a framework or anything like that - just cleaning up.

• Switch to ES6 classes, rather than prototype based classes Slightly cleanup 2 js files #1382
• Use babel so we can consistently use JS features without having to constantly figure out browser support Use babel to transform JS #1380
• Use const/let over var for easier to understand scoping Replace all uses of var with const/let #1376
• Use eslint to lint our code base and catch errors Lint our JS with a generous eslint #1377
• Consider using prettier for style formatting Add pre-commit: pretter, black, pyupgrade, and misc #1381
• Write some unit tests

[manics]

I'm in favour of using prettier and black throughout this repo with pre-commit for autoformatting (code, javascript, markdown, yml). Main thing is to decide when to make the change to avoid conflicts with pending PRs.

I know you've previously said pre-commit is too complicated but it doesn't have to be, just pre-commit run -a is all I ever do, I never bother with the automated hooks 😃

I'm happy to make that change along with docs if you want.

[consideRatio]

Yeah I've been very very happy with pre-commit since a while back, the experience got even better with pre-commit.ci.

prettier, black, and pyupgrade are amazing!