Copy output of a JavaScript variable to the clipboard

OK, I found some time and followed the suggestion by Teemu and I was able to get exactly what I wanted.

So here is the final code for anyone that might be interested. For clarification, this code gets all checked checkboxes of a certain ID, outputs them in an array, named here checkbx, and then copies their unique name to the clipboard.

JavaScript function:

function getSelectedCheckboxes(chkboxName) {
  var checkbx = [];
  var chkboxes = document.getElementsByName(chkboxName);
  var nr_chkboxes = chkboxes.length;
  for(var i=0; i<nr_chkboxes; i++) {
    if(chkboxes[i].type == 'checkbox' && chkboxes[i].checked == true) checkbx.push(chkboxes[i].value);
  }
  checkbx.toString();

  // Create a dummy input to copy the string array inside it
  var dummy = document.createElement("input");

  // Add it to the document
  document.body.appendChild(dummy);

  // Set its ID
  dummy.setAttribute("id", "dummy_id");

  // Output the array into it
  document.getElementById("dummy_id").value=checkbx;

  // Select it
  dummy.select();

  // Copy its contents
  document.execCommand("copy");

  // Remove it as its not needed anymore
  document.body.removeChild(dummy);
}

And its HTML call:

<button id="btn_test" type="button" onclick="getSelectedCheckboxes('ID_of_chkbxs_selected')">Copy</button>

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);
}

For general purposes of copying any text to the clipboard, I wrote the following function:

function textToClipboard (text) {
    var dummy = document.createElement("textarea");
    document.body.appendChild(dummy);
    dummy.value = text;
    dummy.select();
    document.execCommand("copy");
    document.body.removeChild(dummy);
}

The value of the parameter is inserted into value of a newly created <textarea>, which is then selected, its value is copied to the clipboard and then it gets removed from the document.

When you need to copy a variable to the clipboard in the Chrome dev console, you can simply use the copy() command.

https://developers.google.com/web/tools/chrome-devtools/console/command-line-reference#copyobject

I managed to copy text to the clipboard (without showing any text boxes) by adding a hidden input element to body, i.e.:

 function copy(txt){
  var cb = document.getElementById("cb");
  cb.value = txt;
  cb.style.display='block';
  cb.select();
  document.execCommand('copy');
  cb.style.display='none';
 }

<button onclick="copy('Hello Clipboard!')"> copy </button>
<input id="cb" type="text" hidden>

I just want to add, if someone wants to copy two different inputs to clipboard. I also used the technique of putting it to a variable then put the text of the variable from the two inputs into a text area.

Note: the code below is from a user asking how to copy multiple user inputs into clipboard. I just fixed it to work correctly. So expect some old style like the use of var instead of let or const. I also recommend to use addEventListener for the button.

    function doCopy() {

        try{
            var unique = document.querySelectorAll('.unique');
            var msg ="";

            unique.forEach(function (unique) {
                msg+=unique.value;
            });

            var temp =document.createElement("textarea");
            var tempMsg = document.createTextNode(msg);
            temp.appendChild(tempMsg);

            document.body.appendChild(temp);
            temp.select();
            document.execCommand("copy");
            document.body.removeChild(temp);
            console.log("Success!")


        }
        catch(err) {

            console.log("There was an error copying");
        }
    }

<input type="text" class="unique" size="9" value="SESA / D-ID:" readonly/>
<input type="text" class="unique" size="18" value="">
<button id="copybtn" onclick="doCopy()"> Copy to clipboard </button>

function copyToClipboard(text) {
    var dummy = document.createElement("textarea");
    // to avoid breaking orgain page when copying more words
    // cant copy when adding below this code
    // dummy.style.display = 'none'
    document.body.appendChild(dummy);
    //Be careful if you use texarea. setAttribute('value', value), which works with "input" does not work with "textarea". – Eduard
    dummy.value = text;
    dummy.select();
    document.execCommand("copy");
    document.body.removeChild(dummy);
}
copyToClipboard('hello world')
copyToClipboard('hello\nworld')

At the time of writing, setting display:none on the element didn't work for me. Setting the element's width and height to 0 did not work either. So the element has to be at least 1px in width for this to work.

The following example worked in Chrome and Firefox:

    const str = 'Copy me';
    const el = document.createElement("input");
    // Does not work:
    // dummy.style.display = "none";
    el.style.height = '0px';
    // Does not work:
    // el.style.width = '0px';
    el.style.width = '1px';
    document.body.appendChild(el);
    el.value = str;
    el.select();
    document.execCommand("copy");
    document.body.removeChild(el);

I'd like to add that I can see why the browsers are trying to prevent this hackish approach. It's better to openly show the content you are going copy into the user's browser. But sometimes there are design requirements, we can't change.