Javascript String rpn()

String.prototype.rpn = function() {
 var rpn = this.split(' '),
  operators = '+-/*',
  i,//from  w w  w .j  av a  2s.  co m
  o,
  r,
  stack = [];

 for (i = 0; i < rpn.length; i++) {
  o = parseFloat(rpn[i], 10) || rpn[i];

  var stackLength = stack.length;

  if (typeof o == 'string' && operators.indexOf(o) > -1 && stackLength > 1) {
   r = eval('stack[stackLength-2]' + o + 'stack[stackLength-1]');

   stack.splice(stackLength - 2, 2);
   stack.push(r);
  } else {
   stack.push(o);
  }
 }

 if (stack.length == 1) {
  return stack[0];
 } else {
  console.log('ERROR, here\'s your stack:', stack);
  return false;
 }
};