function copyToClipboard(text) {
   const elem = document.createElement('textarea');
   elem.value = text;
   document.body.appendChild(elem);
   elem.select();
   document.execCommand('copy');
   document.body.removeChild(elem);
}

//As simple as this
navigator.clipboard.writeText("Hello World");

navigator.clipboard.writeText('the text')

function copy() {
  var copyText = document.querySelector("#input");
  copyText.select();
  document.execCommand("copy");
}

document.querySelector("#copy").addEventListener("click", copy);

Also works on safari!
  
function copyToClipboard() {
    var copyText = document.getElementById("share-link");
    copyText.select();
    copyText.setSelectionRange(0, 99999);
    document.execCommand("copy");
}

navigator.clipboard.writeText('some text');
// the document must be focused first (call .focus() on a button/input...)