From: https://hackernoon.com/copying-text-to-clipboard-with-javascript-df4d4988697f

copy.js

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
  }
};