First introduction to the concept of objects in JS

Object

1. First introduction to object

JavaScript An object is an unordered collection of keys and values, for example:

var person
 = {
    name: 'zhangsan',
    age: 20,
    tags: ['js','web','mobile'],
    city: 'Beijing',
hit:null
};

The keys of JavaScript objects are all string types, and the values ​​can be any data. type.

# Its properties can contain basic values, objects or functions. An object is actually a set of values ​​without order. We can imagine objects in JS as key-value pairs, where the values ​​can be data and functions.

Object behavior and characteristics

Characteristics - Properties

Behavior - Method

2. If a variable belongs to the object, then the variable can be called It is an attribute of the object. Attributes are generally nouns used to describe the characteristics of things;

#If a function belongs to the object, then the function can be called It is a method of the object. Method is a verb, describing the behavior and function of things

3.new keyword:

The constructor is a special function that is mainly used to initialize the object when creating it, that is, to assign initial values ​​to the object member variables. It is always used together with the new operator in the statement that creates the object. middle.

#1) The constructor is used to create a type of object, the first letter should be capitalized;

2) The constructor only makes sense when used with new.

new will do four things when executed

a.new will do Create a new empty object in memory

b. Let this point to this new object

c. Purpose of executing the constructor: in order to add attributes and methods to new properties of this object

d.new will return this object

Custom constructor

function Person(name,age,job){
this.name = name;
this.age = age;
this.job = job;
this.sayHi = function(){
console.log('Hello,everyBody');
}
}
var p1 = new Person('张三'，22，'actor','Beijing');

new creates a new empty object p1 Zhang San’s space, Call the execution function constructor to add attributes and methods to the p1 object;

4.this关键词

js中this的指向问题，有时候会让人难以捉摸，函数内部的this有几个特点：

1）函数在定义的时候this是不确定的，只有调用的时候才能确定；

2）一般函数直接执行，内部this指向全局window；

3）函数作为一个对象的方法，被该对象所调用，那么this指向的是该对象；

4）构造函数中的this其实是一个隐式对象，类似一个初始化的模型，所有方法和属性都挂载到了这个隐式对象身上，后续通过new关键字来调用，从而实现实例化。

遍历对象：

for(var key in obj){
console.log(obj[key]);
}

打印出来第一个obj[i]是

(对象的下标)

打印出来的第一个obj是遍历过i的对象

for(var key in obj){
console.log(key+"=="+obj[key])

}打印出的结果是：

key是下标0,1,2,3,4,5,6...

obj[key]是遍历对应的下标对象的值

如果打印的是obj.key，因为.key是找对象的属性，该对象没有key属性所以是undefined;

取消对象属性,如果给对象定义key属性并赋值，想要取消key属性

delete obj.key;

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