We actually started with CoffeeScript 15 months ago, because TypeScript wasn't quite far along yet (I don't quite remember why, but we definitely considered it and said "maybe later"). We also started by writing our own scripting language for games... and then after a few months of development we converted the whole codebase to TypeScript and we haven't looked back ^_^

Seems like Phaser's developer initially considered TypeScript but felt it was too early at the time too: http://www.html5gamedevs.com/topic/13599-reconsidering-build.... It looks like he's decided to go with Babel for Phaser 3 / Lazer: https://github.com/photonstorm/lazer