Method to determine whether window.onload is used multiple times_javascript skills

I believe everyone has used window.onload. It is executed when the window is loading. How to judge if window.onload is used multiple times? Take a look at the code below

<script>
function onloadCallback(){
//Todo

}
if (window.onload) {
var onload_random = 'onload'+Math.random();
window[onload_random] = window.onload;
window.onload = function (){
window[onload_random]();
onloadCallback();
};
}
else {
window.onload = function (){
onloadCallback();
};
}
</script>