Here’s what I came up with: http://codepen.io/codeslo_matt/pen/zBgqYB
Calculators are interesting because the buttons are contextual. For instance, if you press a four, you expect to see a four displayed, and if you then press ‘+’ you’d basically expect it to not affect the display at all. But if you press another number and then his ‘+’ again, you’d expect to see a total. What happens if you hit ‘+’ more than once? Or if you hit ‘+’ then ‘-’? What if your answer overflows the display, or if you just type enough numbers to overflow the display? What if you try to divide by zero?
Another thing I added was a clear entry button. This deletes only the most recent number used, and changes both the display and the logging that the challenge specifies.