Basic usage of JS objects (complex data types)

Object characteristics

definition

• Unordered data collection
• Collection of key-value pairs

Key name (attribute name)

• The key name (attribute name) is a string and can contain any characters
• Quotation marks can be omitted. After omitting, only identifiers can be written. If the quotation marks are omitted, the key name is still a string
• All attribute names without [] will automatically become strings. If the value is not a string, it will automatically become a string
• Added [] will be evaluated as a variable (use the variable as the attribute name)
• Object.keys(obj) can get all keys of obj

Hidden properties of objects

1. Every object in JS has a hidden attribute (each object is an unawakened Saiyan (__proto__), he can go to Saiyan planet ( the object's common attribute ) to get himself when he needs to be awakened Something needed ( call ))
2. This hidden attribute stores the address of an object composed of its common attributes
3. This object composed of common properties is called prototype
4. In other words, the hidden attribute stores the address of the prototype

1. Two syntaxes for declaring objects

let obj = {
    
     'name': 'frank', 'age': 18 }
let obj = new Object({
    
    'name': 'frank'})

Second, how to delete the attributes of the object

delete obj.xxx   或 
delete obj['xxx']
即可删除obj的xxx属性

区分[属性值为undefined] 和 [不含属性名]
[含有属性名，属性值为undefined]
'xxx' in obj && obj.xxx === undefined

[不含属性名]
'xxx' in obj === false

注意obj.xxx === undefined
不能断定'xxx'是否为obj的属性

Three, how to view the properties of the object

查看自身所有属性
Objext.keys(obj)

查看自身+共有属性
console.dir(obj)
或者自己一次用Object.keys打印出onj.__proto__

判断一个属性是自身的还是共有的
obj.hasOwnProperty('toString')

查看属性
中括号语法：obj['key']
点语法：obj.key
坑人语法：obj[key]//变量key值一般不为'key'

优先使用中括号语法，进阶用点语法

name是字符串
obj.name 
obj['name']

name是变量
obj[name]

Fourth, how to modify or increase the attributes of the object

•直接赋值
let obj = {
    
    name: 'frank'} // name 是字符串
obj.name = 'frank' // name 是字符串
obj['name'] = 'frank' 
obj[name] = 'frank' // 错，因name 值不确定
obj['na'+'me'] = 'frank'
let key = 'name'; obj[key] = 'frank'
let key = 'name'; obj.key = 'frank' // 错
因为 obj.key 等价于 obj['key']

•批量赋值
Object.assign(obj, {
    
    age: 18, gender: 'man'})

无法通过自身修改或增加共有属性（原型）
let.obj = {
    
    }, obj = {
    
    } //共有toString
obj.toString = 'xxx'只会在改obj自身属性
obj2.toString还是在原型上

修改或增加原型上的属性
obj.__proto__.toString = 'xxx' // 不推荐用__proto__
Object.prototype.toString = 'xxx'
一般来说，不要修改原型，会引起很多问题

修改隐藏属性__proto__
·不推荐使用
let obj = {
    
    name: 'frank'}
let obj2 = {
    
    name: 'jack'}
let common = {
    
    kind: 'human'}
obj.__proto__ = common
obj2.__proto__ = common
注：所有 __proto__ 代码都是强烈不推荐写的

·推荐使用Object.create
let obj = Object.create(common)
obhj.name = 'frank'
let obj2 = Object.create(common)
obj2.name = 'jack'
规范大概的意思是，要改就一开始改，不要后来再改

Five, the difference between'name' in obj and obj.hasOwnProperty('name')

'name' in obj
判断一个对象属性或原型里面是否包含某个key，key为字符串
如果一个属性是从原型链上继承来的，in运算符也会返回 true。

obj.hasOwnProperty('name') 
所有继承了Object的对象都会继承到hasOwnProperty方法。
这个方法可以用于检测一个对象是否包含特定的固有属性；
和in运算符不同，该方法会忽略掉那些从原型链上继承到的属性。