Switch from old javascript to new javascript

[martinRenou]

We should maybe switch from old javascript to new javascript. See http://es6-features.org/.

• var -> let
• old class definition -> new class definition http://es6-features.org/#ClassDefinition

[stefanoborini]

What's the support of ECMA 6 for all browsers we support?

[martinRenou]

If I'm correct, using babelify will assure support with all browsers