MediaWiki Developer's Handbook/Add JavaScript/Predefined functions

This is wildly out-of-date, both these hooks are deprecated. Instead you use ResourceLoader to load JavaScript resources which typically use jQuery to run code at the appropriate event.

addOnloadHook()

addOnloadHook(someFunction);

This hooks onto the completion of the page load, and runs the specified function. You can run an anonymous function too:

addOnloadHook(function(){
    ...
});

addHandler()

addHandler(domElement,'action',someFunction);

This hooks onto the action being performed on some DOM element, running the function. This can be useful for example if you want to change the user's input when submitting a form:

function reallyChangeInput(){
    ...
}
function changeInput() {
    form = document.getElementById('editform');
    if (!form) return false;
    addHandler(form,'submit',reallyChangeInput);
}
addOnloadHook(changeInput);