How to Add onLoad JavaScript event in SharePoint?

SharePoint provides a JavaScript array “_spBodyOnLoadFunctionNames”, any function to be executed onLoad needs to be added to this array e.g.

_spBodyOnLoadFunctionNames.push("ExecuteMyFunctionOnLoad");

Now, why does your JavaScript function doesn’t execute if you just register it
with ClientScript.RegisterStartupScript?

Actually, content pages can’t execute JavaScript function on body load, reason; content pages can’t directly add a function to the body’s OnLoad event if master page contains the <body> element (which is mostly true).

This array is basically a part of init.js located in “C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS\1033\”. If you open the javascript file, you will notice the page onLoad event is handled by function _spBodyOnLoadWrapper, which further calls ProcessDefaultOnLoad, and this function executes all the function names added in array "_spBodyOnLoadFunctionNames".

function addLoadEvent(func) {
var oldonload = window.onload;
if (typeof window.onload != 'function') {
window.onload = func;
} else {
window.onload = function() {
oldonload();
func();
}}}
addLoadEvent(nameOfSomeFunctionToRunOnPageLoad);
addLoadEvent(function() {
/* more code to run on page load */
}
);

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