Nice.

Regarding var result = 0;
You can just declare result variable. IMHO this is better option for readability of your code.
var result;
Or you can initialize it to default value and loose else block.
var result = null;

You could improve formatting. Solution from slaven.m is example of good formatting.

else if(o === "/" && b === 0) { 
   result = null;
   }

This part of code will never be executed. Can you think of why?