//for循环正序遍历

for (key in arr) {
    console.log(`Hello, ${arr[key]}!`);
}

//for循环倒序遍历

for (i = arr.length - 1; i >= 0; i--) {
    console.log(`Hello, ${arr[i]}!`);
}

//while循环正序遍历

var n = 0; 
do {
    console.log(`Hello, ${arr[n]}!`);
    n = n + 1;
} while (n < 3)

//while循环倒序遍历

var n = arr.length - 1; 
do {
    console.log(`Hello, ${arr[n]}!`);
    n = n - 1;
} while (n >= 0)