clipboard-copy.js

function onCopy(e) {
    try {
        e.clipboardData.setData('Text', 'tekst do skopiowania');
        e.preventDefault();
    } catch (err) {
        console.error(err);
    }
}

function onClick() {
    try {
        window.document.oncopy = onCopy;
        window.document.execCommand('copy');
    } catch (err) {
        onError();
    } finally {
        window.document.oncopy = undefined;
    }
}

button.addEventListener('click', onClick, false);