Object属性

 ## `.`点运算符后面总跟字符串
```js
obj.name
obj.age
```
## `[ ]`中跟变量、字符串或`Symbol`(Symbol作为对象属性只能用`[ ]`)
```js
let s = 'name'
obj[s] == obj.name
--------------------------
obj['name']
--------------------------
let w = Symbol('name')
obj[w]
```