JAVASCRIPT

navigator.clipboard is undefined

ou can write an all-in-one wrapper function.

if in secure context (https) : use navigator clipboard api
if not : use the 'out of viewport hidden text area' trick
// return a promise
function copyToClipboard(textToCopy) {
    // navigator clipboard api needs a secure context (https)
    if (navigator.clipboard && window.isSecureContext) {
        // navigator clipboard api method'
        return navigator.clipboard.writeText(textToCopy);
    } else {
        // text area method
        let textArea = document.createElement("textarea");
        textArea.value = textToCopy;
        // make the textarea out of viewport
        textArea.style.position = "fixed";
        textArea.style.left = "-999999px";
        textArea.style.top = "-999999px";
        document.body.appendChild(textArea);
        textArea.focus();
        textArea.select();
        return new Promise((res, rej) => {
            // here the magic happens
            document.execCommand('copy') ? res() : rej();
            textArea.remove();
        });
    }
}
use :

copyToClipboard("I'm going to the clipboard !")
    .then(() => console.log('text copied !'))
    .catch(() => console.log('error'));

navigator.clipboard is undefined

On Firefox, HTTPS is required