How to call a JavaScript File and Function in ASP.NET Project

Question

Hi
I have three TextBox.  Id is (txtdate,txtchallan,txtplace) on a ASP.NET  page and have a javascript file(Myjavascript.js).I want
 1: I want only one  function in  "Myjavascript.js"  File  which check  TextBox is  left blank or not .

2:How This javascript file will call on aspx page .

3:how function of javascript will call on ASP.NET controls.

My main moto is to write single function for a particular task and use it on many ASP.NET controls .

Ranjeet Kumar Singh