js获取对象的属性个数

let math = {
    subject: '数学',
    children: [
        { id: 1, title: '数一' }, 
        { id: 2, title: '数二' }, 
        { id: 3, title: '数三' }, 
        { id: 0, title: '待定' }
    ]
}
// 取对象的属性个数
Object.getOwnPropertyNames(math).length;
Object.keys(math).length;