Validations using Javascript!

Question

Dear friends! I am developing a Web application in ASP.net using VB as language. I want to call javascript function from aspx.I know validations can be done using validation controls but my client wants only the javascript validations. I have a very general problem.. Like If i am showing some data in datagrid with a checkbox to select. Now i want to test whether any checkbox is checked or not and depending upon the codition I have to call a javascript function. Please help. Amol