jquery document ready in plain js

document.js

Document Ready check
This snippet will covers all the browser to check if the DOM is ready.

document.addEventListener("DOMContentLoaded", function() {
   console.log('Your document is ready!');
});
Document Ready check for IE8
Offcourse there is IE8, who wants a different way.

document.attachEvent("onreadystatechange", function(){
 if (document.readyState === "complete"){
   console.log('Your document is ready!');
 }
});
Cross-browser Document Ready check
If I would create a small module of it, it would be like this:

var domIsReady = (function(domIsReady) {
   var isBrowserIeOrNot = function() {
      return (!document.attachEvent || typeof document.attachEvent === "undefined" ? 'not-ie' : 'ie');
   }

   domIsReady = function(callback) {
      if(callback && typeof callback === 'function'){
         if(isBrowserIeOrNot() !== 'ie') {
            document.addEventListener("DOMContentLoaded", function() {
               return callback();
            });
         } else {
            document.attachEvent("onreadystatechange", function() {
               if(document.readyState === "complete") {
                  return callback();
               }
            });
         }
      } else {
         console.error('The callback is not a function!');
      }
   }

   return domIsReady;
})(domIsReady || {});

source
https://www.competa.com/blog/cross-browser-document-ready-with-vanilla-javascript/