Reverse Polish Notation Math Expression Parser

rpn

function assert(res, expected) {
  var result = rpn(res);
  if (result !== expected) {
    throw new Error('Not equals: ', result);
  }
}
function apply(operator, opd1, opd2) {
  var str = 'return ' + opd1 + operator + opd2 + ';';
  return (new Function(str))();
}
function isOperator(char) {
  return "*/+-".indexOf(char) > -1;
}
function rpn(str) {
  function rpnRecur(stack) {
    var last = stack.pop();
    if (isOperator(last)) {
      var oper = last;
      var opd1 = rpnRecur(stack);
      var opd2 = rpnRecur(stack);
      stack.push(apply(oper, opd1, opd2));
      return rpnRecur(stack);
    } else {
      return +last;
    }
  }

  var ast = str.split(' ');
  return rpnRecur(ast);
}

assert('1', 1);
assert('1 2 +', 3);
assert('1 2 -', 1);
assert('1 2 3 + +', 6);
assert('1 2 + 3 + 4 +', 10);
assert('1 2 + 3 + 4 -', -2);
assert('2 2 *', 4);