I’m developing a iPhone app using PhoneGap. In the app there is a page contains only one textarea for user’s input. As there is no other input field on this page, I want to hide the toolbar above the keyboard to give user more free space. Is there any way to do so? using javascript

I don’t know whether this is directly possible with phonegap API but you can change this behavior with native code by writing a PhoneGap plugin to do that.
Refer below links for more detail (they are not exact but will give you the required detail):
Programmatically align a toolbar on top of the iPhone keyboard
http://gabriel-tips.blogspot.in/2011/05/input-accessory-view-how-to-add-extra.html