for in 和for的问题

Question

     下面的for (var attr in arr) {}如果换成是普通的for循环会报
     Uncaught TypeError: Cannot read property 'style' of null
     而for in却不会这是怎么回事
     
        hashchange();
        function hashchange() {
            var arr = ["a1","b1"];
            for (var key in num) {
                if (num.hasOwnProperty(key)) {
                    arr.push(num[key]["classHash"]);
                }
            }
            var firstHash = window.location.hash.substring(1) || arr.shift();
            for (var attr in arr) {
                document.querySelector("."+arr[attr]).style.display = "none";
                document.querySelector("."+firstHash).style.display = "block";
            }
        };