The solution offered by amdfan is the best if you are having a lot of users and configuring their browser isn't an option. Though you could test if the clipboard is available and provide a link for changing the settings, if the users are tech savvy. The JavaScript editor TinyMCE follows this approach.
var memory = ''; // Outside the functions but within the script tag.
function moz_stringCopy(DOMEle, firstPos, secondPos) {
var copiedString = DOMEle.value.slice(firstPos, secondPos);
memory = copiedString;
}
function moz_stringPaste(DOMEle, newpos) {
DOMEle.value = DOMEle.value.slice(0, newpos) + memory + DOMEle.value.slice(newpos);
}
function copyText(text){
function selectElementText(element) {
if (document.selection) {
var range = document.body.createTextRange();
range.moveToElementText(element);
range.select();
} else if (window.getSelection) {
var range = document.createRange();
range.selectNode(element);
window.getSelection().removeAllRanges();
window.getSelection().addRange(range);
}
}
var element = document.createElement('DIV');
element.textContent = text;
document.body.appendChild(element);
selectElementText(element);
document.execCommand('copy');
element.remove();
}
var txt = document.getElementById('txt');
var btn = document.getElementById('btn');
btn.addEventListener('click', function(){
copyText(txt.value);
})
<input id="txt" value="Hello World!" />
<button id="btn">Copy To Clipboard</button>