JavaScript Event Handler

Event handler allows specified Javascript function to be called when an event such as mouse click, happens.  Event handler must be hung onto a DOM tree.  For example,

var button = document.geElementById("submitbutton");
button.onclick = function() {...};

Event handler always starts with "on-something".  Each element can only has 1 event handler attached to it.  If you need to have 2 event handlers, for instance to validate the form and submit to server, you need to use event listener instead.