[javascript] Copy output of a JavaScript variable to the clipboard

I have no knowledge of JavaScript, but I managed to put this code together using bits and bolts from various Stack Overflow answers. It works OK, and it outputs an array of all selected checkboxes in a document via an alert box.

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);
  }
  return checkbx;
}

And to call it I use:

<button id="btn_test" type="button" >Check</button>
<script>
    document.getElementById('btn_test').onclick = function() {
        var checkedBoxes = getSelectedCheckboxes("my_id");
        alert(checkedBoxes);
    }
</script>

Now I would like to modify it so when I click the btn_test button the output array checkbx is copied to the clipboard. I tried adding:

checkbx = document.execCommand("copy");

or

checkbx.execCommand("copy");

at the end of the function and then calling it like:

<button id="btn_test" type="button" onclick="getSelectedCheckboxes('my_id')">Check</button>

But it does not work. No data is copied to clipboard.

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){_x000D_
  var cb = document.getElementById("cb");_x000D_
  cb.value = txt;_x000D_
  cb.style.display='block';_x000D_
  cb.select();_x000D_
  document.execCommand('copy');_x000D_
  cb.style.display='none';_x000D_
 }

<button onclick="copy('Hello Clipboard!')"> copy </button>_x000D_
<input id="cb" type="text" hidden>

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')

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 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() {_x000D_
_x000D_
        try{_x000D_
            var unique = document.querySelectorAll('.unique');_x000D_
            var msg ="";_x000D_
_x000D_
            unique.forEach(function (unique) {_x000D_
                msg+=unique.value;_x000D_
            });_x000D_
_x000D_
            var temp =document.createElement("textarea");_x000D_
            var tempMsg = document.createTextNode(msg);_x000D_
            temp.appendChild(tempMsg);_x000D_
_x000D_
            document.body.appendChild(temp);_x000D_
            temp.select();_x000D_
            document.execCommand("copy");_x000D_
            document.body.removeChild(temp);_x000D_
            console.log("Success!")_x000D_
_x000D_
_x000D_
        }_x000D_
        catch(err) {_x000D_
_x000D_
            console.log("There was an error copying");_x000D_
        }_x000D_
    }

<input type="text" class="unique" size="9" value="SESA / D-ID:" readonly/>_x000D_
<input type="text" class="unique" size="18" value="">_x000D_
<button id="copybtn" onclick="doCopy()"> Copy to clipboard </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.

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.

Nowadays there is a new(ish) API to do this directly. It works on modern browsers and on HTTPS (and localhost) only. Not supported by IE11.

IE11 has its own API.

And the workaround in the accepted answer can be used for unsecure hosts.

function copyToClipboard (text) {
  if (navigator.clipboard) { // default: modern asynchronous API
    return navigator.clipboard.writeText(text);
  } else if (window.clipboardData && window.clipboardData.setData) {     // for IE11
    window.clipboardData.setData('Text', text);
    return Promise.resolve();
  } else {
    // workaround: create dummy input
    const input = h('input', { type: 'text' });
    input.value = text;
    document.body.append(input);
    input.focus();
    input.select();
    document.execCommand('copy');
    input.remove();
    return Promise.resolve();
  }
}

Note: it uses Hyperscript to create the input element (but should be easy to adapt)

There is no need to make the input invisible, as it is added and removed so fast. Also when hidden (even using some clever method) some browsers will detect it and prevent the copy operation.