JavaScript中数组去除重复的三种方法

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"]