It wasn't that long ago where you couldn't programmatically copy text to the clipboard from the web without using Flash. But it's getting pretty well supported these days. IE 10+, Chrome 43+, Firefox 41+, and Opera 29+, says Matt Gaunt in writing about it on Google's developer site.
Here's the example from that article:
That article says it's not supported in Safari, but it is as of Safari 10, which only dropped in September.
You don't need a library to do this stuff, as evidenced by the above demo. But, there is one: clipboard.js. It's only 3kb gzipped. The purpose is to give you a bit of a cleaner API to work with, like success and error events, and configuration through
data-* attributes. Here's a demo of that: