Actually IONIC has 2 variants ..
Ionic 1 : Based on Angular 1 (Uses Javascript)
Ionic 2 : Based on Angular 2 (Uses Typescript , as Angular 2 itself uses Typescript and IONIC is simply a mobile based wrapper for Angular applications ).
Typescript is nothing new .. most of the Typescript syntax is ES6 (2015 version of Javascript ). Also, typescript is one of the most used super-set of JavaScript.
So, just program in Typescript and eventually you see that you can also easily write ES6 .