Check if the document is ready

domready.js

function onReady(callbackFn) {
  if (document.readyState !== 'loading') {
    // Document is already ready, call the callback directly
    callbackFn();
  } else if (document.addEventListener) {
    // All modern browsers to register DOMContentLoaded
    document.addEventListener('DOMContentLoaded', callbackFn);
  } else {
    // Old IE browsers
    document.attachEvent('onreadystatechange', function() {
      if (document.readyState === 'complete') {
        callbackFn();
      }
    });
  }
}

onReady(function() {
  // your code here
});