Take note if you're building a "calculator" as most calculator apps use special formats for storing numbers. As an example, open the JavaScript console in your favourite browser and execute 0.1+0.2. The answer you get won't be 0.3, which is what you would expect it to be.

http://dev.opera.com/articles/view/w...real-decimals/
http://tinyurl.com/cemen4z
http://stackoverflow.com/questions/5...nt-math-broken
http://docs.oracle.com/cd/E19957-01/..._goldberg.html

In short, maybe a calculator isn't the best thing to be implementing for a first learning project. On the other hand, it will teach some of the more exotic stuff that JavaScript can trip you up on.