Because it's easier to maintain good code when you are forced to
from the root of your project run:
curl -s jt.io/lint | sh
Mac OS X / Unix only
If for some reason you must commit a JS file without validating/linting it you can always run a commit with the "no-verify" flag:
git commit --no-verify .
However, when doing this remember that there are no flags to remove bad Karma.
If you want to learn more about this, I've written a blog post in my blog here