JavaScript Event getting Lost and not working after PostBack in ASP.Net UserControl

I have created a user control.  Usercontrol contain a image.  Attached a click event on image jusing javascript function.

image.onclick = function (e) { //some code };

Now this user control is added on aspx page. Thsi is working file first time. however when the page is getting postback this event is not working. UserControl EnableViewstate is true. What should be done?