对象

• 是由键-值组成的无序集合

var xiaoming = {
    name: '小明',
    birth: 1990,
    height: 1.70,
    weight: 65,
    score: null
    'middle-school': 'No.1 Middle School'
};

• 访问元素

xiaoming.name; // '小明'
xiaoming['name']; // '小明'
xiaoming['middle-school']; // 'No.1 Middle School'
xiaoming.age; // undefined

• 添加或删除属性

xiaoming.age = 18; // 新增一个age属性
xiaoming.age; // 18
delete xiaoming.age; // 删除age属性
xiaoming.age; // undefined
delete xiaoming['name']; // 删除name属性
xiaoming.name; // undefined
delete xiaoming.school; // 删除一个不存在的school属性也不会报错