for integers parseInt(number1,10) + parseInt(number2,10)
for floats/decimals parseFloat(number1) + parseFloat(number2)
If you're wondering what the second parameter (10) in parseInt is for:
radix [ - the second parameter] Optional. A value between 2 and 36 indicating the base of the number contained in [the first parameter]. If not supplied, strings with a prefix of '0x' are considered hexadecimal and strings with a prefix of '0' are considered octal. All other strings are considered decimal.
- from MicroSoft's JScript documentation
So if you put the string "09" into parseInt without specifying base10 as the radix you'll end up with an octal integer.... I had some crazy fun with this once when I didn't have a monkeys what was going on!