Conditionally calling JavaScript from server-side

I would like to call javascript function from textbox textchanged event if certain condition is met.

Basically, my textbox has date. If date satisfies certain condition, I want to call a javascript function that explicitly clicks a hidden button. I use this hidden button to fire update on the update panel.

But if date is not valid I don't want to call the javascript function.

I have seen Button1.Attributes.Add type of example, but I don't think it fits my need.

Please help.
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