Very useful. I modified it to copy a JavaScript variable value to clipboard:
function copyToClipboard(val){
var dummy = document.createElement("input");
dummy.style.display = 'none';
document.body.appendChild(dummy);
dummy.setAttribute("id", "dummy_id");
document.getElementById("dummy_id").value=val;
dummy.select();
document.execCommand("copy");
document.body.removeChild(dummy);
}