// 1、for遍历
    var arr = [1,2,3];
    var arr2 = [];
    for(var i = 0;i <= arr.length -1;i++){
        arr2.push(arr[i])
    }
    console.log(arr2);
    // 2、slice
    var arr3 = [1, 2, 3];
    var arr4 = arr3.slice(0);
    console.log(arr4);
    // 3、concat()
    var arr5 = [1,2,3];
    var arr6 = arr5.concat();
    console.log(arr6);