javascript-postfix.js

var operators = ['+', '-', '*', '/'];

var string = '4 2 + 3 5 1 -  * +';

var funcs = {
  '+': function (a, b) {
    return b + a;
  },
  '-': function (a, b) {
    return b - a;
  },
  '*': function (a, b) {
    return b * a;
  },
  '/': function (a, b) {
    return b / a;
  }
}

function postfix (str) {
  var len = str.length;
  var stack = [];
  
  for (var i = 0; i <= str.length; i++) {
    var token = str.charAt(i);    
    var isNumber = parseInt(token);
    var isOperator = operators.indexOf(token) !== -1;

    
    if (isNumber) {
      stack.push(parseInt(token));
    }
    
    if (isOperator) {
      var a = Number(stack.pop());
      var b = Number(stack.pop());
      
      var result = funcs[token](a, b);
      stack.push(result);
    }
  }

  return stack;
}

postfix(string);