JavaScript - run a function once document fully loaded, or straightaway if already loaded

gistfile1.js

window.addEventListener("load", WindowIsLoaded, false);
//readystate :: loading --> interactive (document.DOMContentLoaded) --> complete (window.load) 
if (document.readyState === "complete") {
    window.removeEventListener("load", WindowIsLoaded);
    WindowIsLoaded();
}

// usage----------------->
function WindowIsLoaded() {
    // do stuff once document has fully loaded and processed
}

Sometimes it is useful to run a JavaScript function as soon as possible, but not before the document has fully loaded.
This script creates an event listener, but if document is already fully loaded, it will delete the event handler and execute the handler straight away.

document readyState is initially "loading", but once DOM has loaded changes to "interactive" and the document.DOMContentLoaded event fires. Once document has fully loaded, readyState changes to "loaded" and the window.load event fires.

Note that <script> tags will no longer execute properly once readyState has become "loaded".