Java script problem

Hello Sir!!!
Now i am taken third assignment of java script which has
3 things...
1)String concatenation.
2)Checking the palindrome.
3)Simple Calculator.

I am write script for first two.But how to write script for third one Simple calculator i am not understand.Please help me sir...I am design the calculator using HTML and CSS.

you can take help from
http://www.anaesthetist.com/mnm/javascript/calc.htm#fx