javascript run something at page load and on change 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

//put your JS code in
document.onload = function {
	// the code here will run when all the HTML is loaded (but the css and the images may miss)
}

// or in
window.onload = function {
  // the code here will run when the whole page is loaded (including css and images)
}