Vanilla JavaScript for creating a button that allows users to copy a hardcoded element's text content.

CopyToClipboard.js

/* HTML
<p id="text_element">I am the text that the button will allow you to copy.</p>
<button onclick="copyToClipboard('text_element')">
    Copy
</button>
*/

function copyToClipboard(elementId) {
  // Create an auxiliary hidden input.
  // Reason: The hidden aux input will act as a temporary container for our code to select via highlighting and copying since it's easier to do it on an input element.
  var aux = document.createElement("input");

  // Get the text from the element passed into the input
  aux.value = document.getElementById(elementId).innerHTML;

  // Append the aux input to the body
  document.body.appendChild(aux);

  // Highlight the content
  aux.select();

  // Execute the copy command
  document.execCommand("copy");

  // Remove the input from the body
	document.body.removeChild(aux);
}