execCommand(‘copy’)
There is a very new option. It is cross-browser but it will take time until everyone has updated their browser.
It works by using the document.execCommand('copy'); function.
With this function you’ll copy the select text. This will not only work with textareas but with every selected text on the webpage (like in span, p, div, etc.).
Available in Internet Explorer 10+, Chrome 43+, Opera 29+ and Firefox 41+ (see execCommand compatibility here).
Example
// Setup the variables
var textarea = document.getElementById("textarea");
var answer = document.getElementById("copyAnswer");
var copy = document.getElementById("copyBlock");
copy.addEventListener('click', function(e) {
// Select some text (you could also create a range)
textarea.select();
// Use try & catch for unsupported browser
try {
// The important part (copy selected text)
var ok = document.execCommand('copy');
if (ok) answer.innerHTML = 'Copied!';
else answer.innerHTML = 'Unable to copy!';
} catch (err) {
answer.innerHTML = 'Unsupported Browser!';
}
});<textarea id="textarea" rows="6" cols="40">
Lorem ipsum dolor sit amet, eamsemper maiestatis no.
</textarea><br/>
<button id="copyBlock">Click to copy</button> <span id="copyAnswer"></span>