Im currently doing the challenges related to the front end developement (javascript). But I was wondering is there challenges related to arrow(lambda) function or promises or const/var… the new additions to JS I mean.
There are ES6 challenges in the new curriculum, which FCC’s team of volunteer contributors are working hard to get ready for release. You can take a look at what they will be at beta.freecodecamp.org
The current challenges do not teach any of the new ES6 features. What will work in your browser depends on what your browser supports. Putting /*jshint esversion: 6 */ at the top of the editor will stop it from flagging syntax errors for ES6 features.