JavaScript 解决数组查重 问题（三种解决方法）

    var arr2=[];
        for(var i=0;i<arr1.length;i++){
            if(arr2.indexOf(arr1[i])==-1){
                arr2.push(arr1[i])
            }
        }
        console.log(arr2);

    var arr1 = [1, 2, 2, 3, 4, 4, 4, 4];
        var arr2 = [];
        for (var i = 0; i < arr1.length; i++) {
            for (var j = 0; j < arr1.length; j++) {
                if (arr2.includes(arr1[i]) == false) {
                    arr2.push(arr1[i])
                }
            }
        }
        console.log(arr2);

 var arr1 = [1, 2, 2, 3, 4, 4, 4, 4];
        var arr2 = [];
        for (var i = 0; i < arr1.length; i++) {
            for (var j = 0; j < arr1.length; j++) {
                if (arr2.includes(arr1[i]) == false) {
                    arr2.splice(0, 0, arr1[i])
                }
            }
        }
        console.log(arr2);