• Browsing to http://nodejs.org/download/, or
• brew install node

./calculator

$ ./calculator 
? 

Basic maths are supported:

? 3 * 3
=> 9

? 3 + 4 * 3
=> 15

? (3 + 4) * 3
=> 21

It's JavaScript math though (for now), so you can't always trust the results...

? .1 + .2
=>  0.30000000000000004

The CLI supports exponentiation with **:

? 3 * 4**5
=>  3072

It supports variable assignments, one per line. Variable names must be lowercase, and consist only of letters.

? x = 3
=> 3
? x + 3
=> 6

? x = 3
=> 3
? y = 4
=> 4
? x + y
=> 7

? asdf = 3 + 4 ** 7 * 4 + (3 + 4)
=> 65546

You can also call functions. Calls are passed through to functions defined on JavaScript's Math object:

? sin(2)
=>  0.9092974268256817  // (radians)

? pow(3 * 4, 1 + 1)
=> 144

Nested function calls are supported as well:

? pow( sin(2), 3 ** (0 + 1) )
=>  0.7518269446689928

• The error messages thrown by the parser are probably next to worthless most of the time, but if you don't mistype anything, it'll work great!
• The actual evaluation is currently delegating to the JS engine for evaluation, so it's basically double precision floating point math, which is mostly fine. Someday I plan to integrate (probably very slow) arbitrary-precision arithmetic. I have the beginnings of a BigInt class in my projecteuler solutions, but it doesn't handle subtraction/division/negative numbers yet. Switching to aribtrary precision numbers would also eliminate the usefulness of the JS Math function passthrough.
• I think it would be fun to extend to allow the user to define arbitrary functions.