Vanilla JavaScript Document Ready

DOM-Ready.js

Document.prototype.ready = function(callback) {
  if(callback && typeof callback === 'function') {
    document.addEventListener("DOMContentLoaded", function() {
      if(document.readyState === "interactive" || document.readyState === "complete") {
        return callback();
      }
    });
  }
};

// usage
document.ready(function() { alert('ok!'); });