The OnClientSelectedIndexChanging client-side event occurs when the selected item is about to be changed. This event can be cancelled.

The event handler receives two parameters:

  1. The instance of the listbox firing the event

  2. An event arguments parameter containing the following methods:

  • get_item() - returns the item that is about to be selected.

  • set_cancel() - lets you cancel the event and prevent items from being selected.

  • get_domEvent() - returns the DOM event object

The example below shows how to ask the user before selecting the item:

function onClientSelectedIndexChangingHandler(sender, e) {
    if (!confirm("Are you sure?")) {

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