Please use the manual. A simple search with query "clipboard" would have sent you to a node-webkit page manual which states:
The current text that the user has copied to the clipboard, if any. This allows you to 'paste' the data automatically.
Although this will help you only if your game is meant to run on node-webkit. If you're targeting browsers, there is no viable solution supported across browsers, except IE's
I've managed to come across this data with two searches, one on manual, one on google, and i've spent 8 minutes of my time on this post in hope that someone will learn to use the resources given to him.
Please use the search function on the manual, forums and google before asking questions.