vanilla.js document ready event

An event listener when the webpage is fully loaded could be achieved by simply creating a new file like util.js and pasting this simple code inside.

document.onreadystatechange = () => {
    if (document.readyState === 'complete') {
        document.dispatchEvent(new Event("ready"));
    }
};

including my util.js in the html file then in my javascript:

document.addEventListener("ready", () => {

    // javascript launched when the website is fully loaded and the DOM is build.
    
}, { once: true });

I understand that this is not save in all conditions and not suitable for bigger projects. But for my use cases it works just fine.