when document load do something js code example

Example 1: JavaScript that executes after page load

//two ways of executing JS code after page is loaded, use "DOMContentLoaded" when able

document.addEventListener("DOMContentLoaded", function(){
    //dom is fully loaded, but maybe waiting on images & css files
});

window.addEventListener("load", function(){
    //everything is fully loaded, don't use me if you can use DOMContentLoaded
});

Example 2: html tag run only after whole page is loaded

<body onload="script();">
<!-- will call the function script when the body is load -->