Experimenting on Reverse Polish notation.... Competing with @dftcdr and @dark_garret

Reverse Polish Notation. Parser and calculator.

function isOperator(c) {
   return '/*+-^'.indexOf(c) != -1;
}

function peek(stack) {
   return stack[stack.length - 1];
}

var doOper = {
   '+': function(a, b) {
      return a + b;
   },
   '-': function(a, b) {
      return a - b;
   },
   '*': function(a, b) {
      return a * b;
   },
   '/': function(a, b) {
      return a / b;
   },
   '^': function(a, b) {
      return Math.pow(a, b);
   }
};

function parse(_str) {
   _str = _str.replace(/ /g, '');
   var s = '', o = '', c, i = 0, opstack = [], stack = [];
   var priority = {
      '+': 1,
      '-': 1,
      '*': 2,
      '/': 2,
      '^': 3,
      '(': 0
   };
   function endOp() {
      if(o) {
         if(opstack.length) {
            var last = peek(opstack);
            if(priority[o] <= priority[last]) {
               stack.push(opstack.pop());
            }
         }
         opstack.push(o);
         o = '';
      }
   }

   function endNumber() {
      if(s) {
         stack.push(Number(s));
         s = '';
      }
   }

   while(c = _str.charAt(i)) {
      if(c == '(') {
         opstack.push(c);
      } else if(c == ')') {
         endOp(); endNumber();
         if(opstack.length) {
            while(true) {
               var op = opstack.pop();
               if(op) {
                  if(op == '(') {
                     break;
                  } else {
                     stack.push(op);
                  }
               }
               else
                  throw "Unmatched rbrace";
            }
         } else {
            throw "Unmatched rbrace";
         }
      } else if('0' <= c && c <= '9') {
         endOp();
         s += c;
      } else if(c == '.') {
         if(s && s.indexOf('.') == -1) {
            s += c;
         } else {
            throw "Unexpected .";
         }
      } else if(isOperator(c)) {
         endNumber();
         o = c;
         endOp();
      } else
         throw "Unknown symbol " + c;
      i++;
   }
   endOp();
   endNumber();
   while(opstack.length) {
      stack.push(opstack.pop());
   }
   return stack;
}

function calc(stack) {
   var i = 0, c;
   while((c = stack[i]) !== undefined) {
      if(isOperator(c)) {
         stack.splice(
            i - 2,
            3,
            doOper[c](stack[i - 2], stack[i - 1]) );
         i -= 2;
      }
      i++;
   }
   if(stack.length != 1)
      throw "Invalid expression";
   return stack.pop();
}

var ex = '(3*5-6) - ((3-3) * (5-6))';
var stack = parse(ex);
console.log(stack);
console.log(calc(stack));