javascript for-in语句，for-of语句

for-in语句

遍历时的 num 值为数组的索引

let lessons = [
    {title: '媒体查询响应式布局',category: 'css'},
    {title: 'FLEX 弹性盒模型',category: 'css'},
    {title: 'MYSQL多表查询随意操作',category: 'mysql'}
];

for (const num in lessons) {
    console.log(`标题: ${lessons[num].title}`);
}