Load scripts dynamically in JS

loadScript.js

//Source: https://aaronsmith.online/easily-load-an-external-script-using-javascript/

//-------------------PROMISE METHOD --------------------------
/**
 * Loads a JavaScript file and returns a Promise for when it is loaded
 */
const loadScript = src => {
  return new Promise((resolve, reject) => {
    const script = document.createElement('script')
    script.type = 'text/javascript'
    script.onload = resolve
    script.onerror = reject
    script.src = src
    document.head.append(script)
  })
}

loadScript('https://code.jquery.com/jquery-3.4.1.min.js')
  .then(() => loadScript('https://code.jquery.com/ui/1.12.1/jquery-ui.min.js'))
  .then(() => {
    // now safe to use jQuery and jQuery UI, which depends on jQuery
  })
  .catch(() => console.error('Something went wrong.'))
  


  
//-------------------CALLBACK METHOD --------------------------
/**
* Plays well with historic artifacts
*/
function loadScript(src, callback) {

  var script = document.createElement('script')
  script.type = 'text/javascript'

  // IE
  if (script.readyState) {
    script.onreadystatechange = function () {
      if (script.readyState === 'loaded' || script.readyState === 'complete') {
        script.onreadystatechange = null
        callback()
      }
    }
  }
  // Others
  else {
    script.onload = callback
  }

  script.src = src
  document.head.appendChild(script)
}