On onkeyup event the value is still not updated, since it is not applied. The new value gets applied on blur/pressing enter or using set_value client side method.
Before applying the new value, it is considered just as DisplayValue (which can be EmptyMessage/ ErrorMessage/ Formated Text. etc.)
You can still access this text using get_textBoxValue() property during the keyup event. And you can force the control to consider this as actual value using textbox.set_value(textbox.get_textBoxValue());
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