loadScript -- load javascript file from javascript and run a callback function after it's loaded

loadScript.js

function loadScript(url, callback)
{
    // adding the script tag to the head as suggested before
   var head = document.getElementsByTagName('head')[0];
   var script = document.createElement('script');
   script.type = 'text/javascript';
   script.src = url;

   // then bind the event to the callback function 
   // there are several events for cross browser compatibility
   script.onreadystatechange = callback;
   script.onload = callback;

   // fire the loading
   head.appendChild(script);
}

var jqueryIsLoaded = function() {
    alert("jqueryIsLoaded")
};

loadScript("https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.js", jqueryIsLoaded);