Use the Clipboard API!

The simplest modern solution is:

navigator.clipboard.writeText(value)

That value can later be accessed with:

navigator.clipboard.readText()

NOTE: To use in an iframe, you'll need to add write (and maybe read) permissions

<iframe src='' allow='clipboard-read; clipboard-write'/>

NOTE: To use in an browser extension (on a webpage), you'll need either to:

• call from a user triggered event (click...)
• add the 'clipboardWrite' permission to the manifest

NOTE: To use in the dev console, use copy() instead

copy('string')

W3Schools Tutorial

CanIUse

Along with copying the text , you also have to make sure that any previously selected component remains selected after copying to clipboard.

Here's the full code :

const copyToClipboard = str => {
  const el = document.createElement('textarea');  // Create a <textarea> element
  el.value = str;                                 // Set its value to the string that you want copied
  el.setAttribute('readonly', '');                // Make it readonly to be tamper-proof
  el.style.position = 'absolute';                 
  el.style.left = '-9999px';                      // Move outside the screen to make it invisible
  document.body.appendChild(el);                  // Append the <textarea> element to the HTML document
  const selected =            
    document.getSelection().rangeCount > 0        // Check if there is any content selected previously
      ? document.getSelection().getRangeAt(0)     // Store selection if found
      : false;                                    // Mark as false to know no selection existed before
  el.select();                                    // Select the <textarea> content
  document.execCommand('copy');                   // Copy - only works as a result of a user action (e.g. click events)
  document.body.removeChild(el);                  // Remove the <textarea> element
  if (selected) {                                 // If a selection existed before copying
    document.getSelection().removeAllRanges();    // Unselect everything on the HTML document
    document.getSelection().addRange(selected);   // Restore the original selection
  }
};

ps adding the source

HTML:

<button type='button' id='btn'>Copy</button>

JS

document.querySelect('#btn').addEventListener('click', function() {
   copyToClipboard('copy this text');
});

JS / Function:

function copyToClipboard(text) {
    var selected = false;
    var el = document.createElement('textarea');
    el.value = text;
    el.setAttribute('readonly', '');
    el.style.position = 'absolute';
    el.style.left = '-9999px';
    document.body.appendChild(el);
    if (document.getSelection().rangeCount > 0) {
        selected = document.getSelection().getRangeAt(0)
    }
    el.select();
    document.execCommand('copy');
    document.body.removeChild(el);
    if (selected) {
        document.getSelection().removeAllRanges();
        document.getSelection().addRange(selected);
    }
};

guest271314's answer applied to multiple elements:

spans = document.querySelectorAll(".class");
for (const span of spans) {
  span.onclick = function() {
    document.execCommand("copy");
  }

  span.addEventListener("copy", function(event) {
    event.preventDefault();
    if (event.clipboardData) {
      event.clipboardData.setData("text/plain", span.textContent);
      console.log(event.clipboardData.getData("text"))
    }
  });
}

<span class="class">text</span>
<br>
<span class="class">text2</span>

This is the most suitable way to do it. It will copy all text in elements with the class "copy" in them.

var copy = document.querySelectorAll(".copy"); 

for (const copied of copy) { 
  copied.onclick = function() { 
    document.execCommand("copy"); 
  };  
  copied.addEventListener("copy", function(event) { 
    event.preventDefault(); 
    if (event.clipboardData) { 
      event.clipboardData.setData("text/plain", copied.textContent);
      console.log(event.clipboardData.getData("text"))
    };
  });
};

.copy {
            
  cursor: copy;
            
}

<p><span class="copy">Text</span></p>
<p><span class="copy">More Text</span></p>
<p><span class="copy">Even More Text</span></p>