In ASP.NET the page's life cycle cannot be interrupted for calling a client script function and after that resuming the life cycle execution. It will be good if you can switch to a different approach. One possible option is to register a client script containing all the logic for determining whether to show client confirmation and after postback process the user's input on the server. Or you can show the confirmation on the next page load after the first postback to the server (which determines weather to show the confirmation), and then process the user input on the second postback.
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