Basic script loader in JavaScript with optional callback on `load`

script-loader.js

// Loads an external js file and optionally calls a callback on completion
function loadScript(url, callback) {
  var script = document.createElement('script');
  document.body.appendChild(script);
  
  if (typeof callback === 'function') {
    script.addEventListener('load', callback, false);
  }
  script.src = url;
  
  return script;
}