What is a constructor? Detailed explanation of constructors in JavaScript

As the basis of prototype and prototype chain, first understanding the constructor and its execution process can better help us learn the knowledge of prototype and prototype chain. This article will take you to learn more about the constructor in JavaScript and introduce how to use the constructor to create a js object. I hope it will be helpful to you!

#1. What is a constructor?

When an ordinary function is used to create a class object, it is called a constructor, or constructor. (For the convenience of understanding, you can understand the creation of constructors in JavaScript as the creation of classes in other languages. The purpose is to use it to instantiate an object through new)

function Person(){
//...
}
//当做普通函数调用
var obj=Person();

//构造函数调用
	 var obj=new Person();

Characteristics of constructors :

• In terms of writing standards, we are accustomed to capitalizing the first letter of the constructor name.

• Create an object through new.

• There is a return value without writing return inside, and what is returned is an object.

Use the constructor to create a js object

• The constructor creates the object (the method is written in the constructor, disadvantages : Each time the constructor is executed, a method will be created.)

	  function Person(name,age,gender){
            this.name=name;
            this.age=age;
            this.gender=gender;
            // 方法写在里面
            this.sayName=function(){
                 console.log(this.name);
             }      
          }
    
          function Dog(name,age){
              this.name=name;
              this.age=age;
          }
         
          var obj=new Person("张三",18,"男");
          var obj1=new Person("李四",16,"男");
          
          var dog=new Dog("乐乐",2);
          obj.sayName();
          obj1.sayName();
          
          console.log(obj);
          console.log(obj1);
          console.log(dog);

• The constructor creates the object (the method is written outside the constructor, disadvantages: The method is global Method, pollute the global.)

	   function Person(name,age,gender){
            this.name=name;
            this.age=age;
            this.gender=gender;
            this.sayName=fun;   //方法写在外面
          }
    	 function fun(){
            console.log(this.name);
        }
          function Dog(name,age){
              this.name=name;
              this.age=age;
          }
          
          var obj=new Person("张三",18,"男");
          var obj1=new Person("李四",16,"男");
          
          var dog=new Dog("乐乐",2);
          obj.sayName();
          obj1.sayName();
          
          console.log(obj);
          console.log(obj1);
          console.log(dog);

• Constructor function creates object transformation (method is created through prototype object)

Prototype object: prototype

Every function we create, the parser will add a prototype attribute to the function.

Points to the prototype object of the constructor. We can access this property through __proto__.

Constructor.prototype.xxx, xxx can be a variable or a method. During the execution process, it will first look for methods or variables in the object. If it cannot be found, it will look for them in the prototype.

	  function Person(name,age,gender){
            this.name=name;
            this.age=age;
            this.gender=gender;
          }
          function Dog(name,age){
              this.name=name;
              this.age=age;
          }
          
        /*为person添加统一的方法， 到原型对象中*/
        Person.prototype.sayName=function(){
            console.log(this.name);
        }
          var obj=new Person("张三",18,"男");
          var obj1=new Person("李四",16,"男");
          
          var dog=new Dog("乐乐",2);
          obj.sayName();
          obj1.sayName();
          
          console.log(obj);
          console.log(obj1);
          console.log(dog);

Running results:

2. Why use a constructor?

To learn each concept, you must not only know what it is, but also why and what kind of problem it solves.

For example, if we want to enter the personal information of each student in the first grade class, then we can create some objects, such as:

var p1 = { name: 'zs', age: 6, gender: '男', hobby: 'basketball' };
var p2 = { name: 'ls', age: 6, gender: '女', hobby: 'dancing' };
var p3 = { name: 'ww', age: 6, gender: '女', hobby: 'singing' };
var p4 = { name: 'zl', age: 6, gender: '男', hobby: 'football' };
// ...

Like the above, we Each student's information can be treated as an object. However, we will find that we repeatedly write a lot of meaningless code. Such as name, age, gender, hobby. If there are 60 students in this class, we have to write it 60 times.

At this time, the advantages of the constructor are reflected. We found that although each student has attributes such as name, gender, and hobby, they are all different, so we pass these attributes as parameters of the constructor. And since they are all first-year students, their ages are basically 6 years old, so we can write them down and deal with them individually when encountering special situations. At this point, we can create the following function:

function Person(name, gender, hobby) {
    this.name = name;
    this.gender = gender;
    this.hobby = hobby;
    this.age = 6;
}

After creating the above function, we can call it through the new keyword, that is, create the object through the constructor.

var p1 = new Person('zs', '男', 'basketball');
var p2 = new Person('ls', '女', 'dancing');
var p3 = new Person('ww', '女', 'singing');
var p4 = new Person('zl', '男', 'football');
// ...

At this point you will find that creating objects will become very convenient. Therefore, although the process of encapsulating the constructor will be more troublesome, once the encapsulation is successful, it will be very easy for us to create objects, which is why we use constructors.

When using object literals to create a series of objects of the same type, these objects may have some similar characteristics (properties) and behaviors (methods). At this time, a lot of repeated code will be generated, and using constructors You can achieve code reuse.

3. The execution process of the constructor

Let’s talk about some basic concepts first.

function Animal(color) {
 this.color = color;
}

When a function is created, we don't know whether it is a constructor. Even if the function name is in capital letters like the example above, we can't be sure. Only when a function is called with the new keyword can we say that it is a constructor. Like the following:

var dog = new Animal("black");

Below we only discuss the execution process of the constructor, that is, when it is called with the new keyword.

We still take the above Person as an example.

function Person(name, gender, hobby) {
 this.name = name;
 this.gender = gender;
 this.hobby = hobby;
 this.age = 6;
}

var p1 = new Person('zs', '男', 'basketball');

At this time, the constructor will have the following execution processes:

1) When called with the new keyword, a new memory space will be created, marked as an instance of Animal.

2) This inside the function body points to the memory

Through the above two steps, we can draw this conclusion.

var p2 = new Person('ls', '女', 'dancing');  // 创建一个新的内存 #f2
var p3 = new Person('ww', '女', 'singing');  // 创建一个新的内存 #f3

Whenever an instance is created, a new memory space (#f2, #f3) will be created. When #f2 is created, this inside the function body points to #f2, and #f3 is created. At this time, this inside the function body points to #f3.

3） 执行函数体内的代码
通过上面的讲解，你就可以知道，给 this 添加属性，就相当于给实例添加属性。

4）默认返回 this

由于函数体内部的this指向新创建的内存空间，默认返回 this ，就相当于默认返回了该内存空间，也就是上图中的 #f1。此时，#f1的内存空间被变量p1所接受。也就是说 p1 这个变量，保存的内存地址就是 #f1，同时被标记为 Person 的实例。

以上就是构造函数的整个执行过程。

4、构造函数的返回值

构造函数执行过程的最后一步是默认返回 this 。言外之意，构造函数的返回值还有其它情况。下面我们就来聊聊关于构造函数返回值的问题。

1）没有手动添加返回值，默认返回 this

function Person1() {
 this.name = 'zhangsan';
}

var p1 = new Person1();

按照上面讲的，我们复习一遍。首先，当用 new 关键字调用时，产生一个新的内存空间 #f11，并标记为 Person1 的实例；接着，函数体内部的 this 指向该内存空间 #f11；执行函数体内部的代码；由于函数体内部的this 指向该内存空间，而该内存空间又被变量 p1 所接收，所以 p1 中就会有一个 name 属性，属性值为 ‘zhangsan’。

p1: {
 name: 'zhangsan'
}

2）手动添加一个基本数据类型的返回值，最终还是返回 this

function Person2() {
 this.age = 28;
 return 50;
}

var p2 = new Person2();
console.log(p2.age);   // 28
p2: {
 age: 28
}

如果上面是一个普通函数的调用，那么返回值就是 50。

3）手动添加一个复杂数据类型(对象)的返回值，最终返回该对象

直接上例子

function Person3() {
 this.height = '180';
 return ['a', 'b', 'c'];
}

var p3 = new Person3();
console.log(p3.height);  // undefined
console.log(p3.length);  // 3
console.log(p3[0]);      // 'a'

再来一个例子

function Person4() {
  this.gender = '男';
  return { gender: '中性' };
}

var p4 = new Person4();
console.log(p4.gender);  // '中性'

5、构造函数首字母必须大写吗？

大小写都可以

6、不用new关键字，直接运行构造函数，是否会出错？

如果不会出错，那么，用new和不用new调用构造函数，有什么区别？

1）使用new操作符调用函数

例子：

function Person(name){
  this.name = name;
  this.say = function(){
    return "I am " + this.name;
  }
}

var person1 = new Person('nicole');
person1.say(); // "I am nicole"

用new调用构造函数，函数内部会发生如下变化：

创建一个this变量，该变量指向一个空对象。并且该对象继承函数的原型；
属性和方法被加入到this引用的对象中；
隐式返回this对象（如果没有显性返回其他对象）
用伪程序来展示上述变化：

function Person(name){
  // 创建this变量，指向空对象
  var this = {}; 
  // 属性和方法被加入到this引用的对象中
  this.name = name;
  this.say = function(){
    return "I am " + this.name;
  }
  // 返回this对象
  return this;
}

可以看出，用new调用构造函数，最大特点为，this对象指向构造函数生成的对象，所以，person1.say()会返回字符串： “I am nicole”。

小贴士：如果指定了返回对象，那么，this对象可能被丢失。

function Person(name){
  this.name = name;
  this.say = function(){
    return "I am " + this.name;
  }
  var that = {};
  that.name = "It is that!";
  return that;
}

var person1 = new Person('nicole');
person1.name; // "It is that!"

2）直接调用函数

如果直接调用函数，那么，this对象指向window，并且，不会默认返回任何对象（除非显性声明返回值）。

还是拿Person函数为例，直接调用Person函数：

var person1 = Person('nicole');
person1; // undefined
window.name; // nicole

可见，直接调用构造函数的结果，并不是我们想要的。

3）小结

为了防止因为忘记使用new关键字而调用构造函数，可以加一些判断条件强行调用new关键字，代码如下：

function Person(name){
  if (!(this instanceof Person)) {
    return new Person(name);
  }
  this.name = name;
  this.say = function(){
    return "I am " + this.name;
  }
}

var person1 = Person('nicole');
console.log(person1.say()); // I am nicole
var person2 = new Person('lisa');
console.log(person2.say()); // I am lisa

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