Vanilla JS alternative to JQuery's document ready function

gistfile1.js

var callback = function () {
// The code to be executed after DOM is ready
}

if ( document.readyState === "complete" ||
    (document.readyState !== "loading" && !document.documentElement.doScroll) ) {
    callback();
} else {
    document.addEventListener("DOMContentLoaded", callback);
}