From the Firebase tutorial, a good javascript ide is https://atom.io

• open source, free, runs js.
• using http://electron.atom.io to build "cross platforms apps using html+js"

Javascript not-quite-replacements:

• CoffeeScript: http://coffeescript.org … compiles 1-to-1 to vanilla JS.
• TypeScript: https://www.typescriptlang.org … also a compiler. More JS like, with decoration for types and access.
• - ES6: ...
• The high level summary is that CoffeeScript is Ruby/Python like and TypeScript is .Net like in their sugar-syntax addition/replacement.
• TypeScript seems to have a VS.Net add-on (confirm?)
• In terms of age: CoffeeScript < TypeScript < ES6.

Point of view: CoffeeScript is different enough that it's a new habit to learn. TypeScript is JS plus some sugar so can be used to decorate existing JS stuff. ES6 is the way to go but what about support? Compatibility chart here: ES6 supported in V8 since Chrome 42 (so also in Node.js). Mostly there in recent Firefox and MS Edge.

Bottom line: don't bother with CS/TS. Go with ES6 where possible.