ASP.NET - read only for a particular column using javascript

I have a html table named "employee". In that table i have four columns named "empid" , "emptype","position","empcode". I am populating this table from the DB.

The emptype column will populate the value as "permanent" or "contract".

If the text of the "emptype" column is contract, the position and empcode  columns against that empid should be readonly/disabled.

I have to do this using java script.

Can any one help me out and share the code plz.

Thanks a lot in advance