Your event listener is too restrictive. When the user pastes her text via the context menu or drags and drops it on the input, the button would not be enabled.
Furthermore, your code is unnecessarily verbose.
Take a look at this JSBin for a better version:
http://jsbin.com/cevod/2/edit?js,output
sbruchmann 3670 days ago. link 1 point ▲ ▼