It's good you've found working solution. Take a look at Matija7's answer to see cleaner solution.

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;

However, you could improve formatting.
result= a + b; => result = a + b;

Closing if and elese if block brace should be aligned with block key word.

else if(o === "-") {
  result = a - b;
  }

else if(o === "-") {
  result = a - b;
}

You don't need last return statement return result; since all your if statement blocks return value;