JS中數組去除重複

function outRepeat(a){
            var hash=[],arr=[];
            for (var i = 0; i < a.length; i++) {
                hash[a[i]]!=null;
                if(!hash[a[i]]){
                    arr.push(a[i]);
                    hash[a[i]]=true;
                }
            }
            console.log(arr);
        }
        outRepeat([2,4,4,5,"a","a"]);//[2, 4, 5, "a"]

function outRepeat(a){
            var hash=[],arr=[];
            for (var i = 0,elem;(elem=a[i])!=null; i++) {
                if(!hash[elem]){
                    arr.push(elem);
                    hash[elem]=true;
                }
            }
            console.log(arr);
        }
        outRepeat([2,4,4,5,"a","a"]);//[2, 4, 5, "a"]

function outRepeat(a){
            var hash=[],arr=[];
            for (var i = 0; i < a.length; i++) {
                hash[a[i]]=null;
            }
            for(var key in hash){
                   arr.push(key);                
                }
            console.log(arr);
        }
        outRepeat([2,4,4,5,"a","a"]);//["2", "4", "5", "a"]