如何指定数组索引？

let arr = [10, 20, 30, 40, 50];
console.log(arr[2]); // 输出: 30

arr = [10, 20, 30, 40, 50]
print(arr[2])  # 输出: 30

int[] arr = {10, 20, 30, 40, 50};
System.out.println(arr[2]); // 输出: 30

#include <iostream>
using namespace std;

int main() {
    int arr[] = {10, 20, 30, 40, 50};
    cout << arr[2] << endl; // 输出: 30
    return 0;
}

let arr = [10, 20, 30];
if (index >= 0 && index < arr.length) {
    console.log(arr[index]);
} else {
    console.log("索引越界");
}

let arr = [];
arr.push(10); // 添加元素
arr.pop();    // 移除最后一个元素

let matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
];
for (let i = 0; i < matrix.length; i++) {
    for (let j = 0; j < matrix[i].length; j++) {
        console.log(matrix[i][j]);
    }
}