Comparative analysis of several inheritance methods in Javascript_Basic knowledge

Opening
In a 'strict' sense, JavaScript is not a true object-oriented language. The reason for this statement is generally that JavaScript as a weakly typed language is very different from the inheritance method of strong languages ​​​​such as Java or C#, so it is a non-mainstream object-oriented method by default, and there are even many The book describes it as a 'not fully object-oriented' language. In fact, I personally feel that the method is not important, what is important is whether it has object-oriented thinking. People who say that JavaScript is not an object-oriented language may not have studied the inheritance method of JavaScript in depth, so I wrote this article for communication.

Why you need to use javascript to implement inheritance
The performance of early PC machines is really not flattering. All the pressure is on the server side, and the client browser is purely for decoration. Coupled with the popular table layout and telephone line Internet access at that time, browsing a web page was very slow; now the Internet era is developing rapidly, personal computer hardware has been greatly improved, and the performance of client browsers is also very disappointing. The model of web development is also quietly changing: the server is no longer as "hard" as before. Instead, the browser should take on as many tasks as possible. In this way, the pressure is distributed to each client. Not only does the enterprise Saving costs also makes web front-end development more interesting - more and more front-end frameworks are emerging, and even many front-end MVC frameworks have emerged. In this context, the role of JavaScript is definitely not just to do some simple verification, send some requests or operate some DOM. It needs to play more roles like front-end routing and business layer, and JavaScript needs to do a lot of logical tasks. , which includes the abstraction of front-end data (i.e. model), and only by using object-oriented thinking can the abstracted data be processed well, so inheritance is very important here.

Start with a simple need
Now extract a model named Person from the front desk, which has basic attributes name and age. By default, everyone can speak, so the speaking function is placed on the prototype object for each instance to enjoy. . Now for Man, it needs to inherit the basic attributes of Person and add its own unique attributes on this basis.

function Person (name, age) {
  this.name = name;
  this.age = age;
}
Person.prototype.say = function(){
  console.log('hello, my name is ' + this.name);
};
function Man() {
  //my own properties
}

The following introduces several mainstream inheritance methods.

1. Prototype chain inheritance

function Person (name, age) {
  this.name = name;
  this.age = age;
}
Person.prototype.say = function(){
  console.log('hello, my name is ' + this.name);
};
function Man() {
}
Man.prototype = new Person('pursue');
var man1 = new Man();
man1.say(); //hello, my name is pursue
var man2 = new Man();
console.log(man1.say === man2.say);//true
console.log(man1.name === man2.name);//true

This inheritance method is very direct. In order to obtain all attribute methods of Person (instance and prototype), the instance new Person('pursue') of the parent class is directly assigned to the prototype of the subclass. In fact, the subclass The class instances man1 and man2 themselves are completely empty objects. All attributes and methods have to be found on the prototype chain, so the attributes and methods found are the same.
Therefore, it is unrealistic to directly use prototype chain inheritance.

2. Use constructor inheritance

function Person (name, age) {
  this.name = name;
  this.age = age;
}
Person.prototype.say = function(){
  console.log('hello, my name is ' + this.name);
};
function Man(name, age) {
  Person.apply(this, arguments);
}
//Man.prototype = new Person('pursue');
var man1 = new Man('joe');
var man2 = new Man('david');
console.log(man1.name === man2.name);//false
man1.say(); //say is not a function

Here the subclass uses apply in the constructor to call the constructor of the parent class, so as to achieve the effect of inheriting the properties of the parent class. It is much better than directly using the prototype chain. At least each instance has its own share. resources, but this method can only inherit the instance attributes of the parent class, so the say method cannot be found. In order to inherit all the attributes and methods of the parent class, the prototype chain must be modified, thus introducing the combined inheritance method.

3. Combination inheritance

function Person (name, age) {
  this.name = name;
  this.age = age;
}
Person.prototype.say = function(){
  console.log('hello, my name is ' + this.name);
};
function Man(name, age) {
  Person.apply(this, arguments);
}
Man.prototype = new Person();
var man1 = new Man('joe');
var man2 = new Man('david');
console.log(man1.name === man2.name);//false
console.log(man1.say === man2.say);//true
man1.say(); //hello, my name is joe

It should be noted that the instance attributes of man1 and man2 actually override the prototype attributes, but they do not override the say method on the prototype (because they do not have it), so here man1.say === man2.say still returns true, so be careful not to override the prototype property as it is common to all instances.

4. Parasitic combination inheritance

To be honest, I really don’t know the name of the following form, but it is indeed the most popular and classic JavaScript inheritance method. In fact, you only need to understand the structure of the prototype object:

function Person (name, age) {
      this.name = name;
      this.age = age;
    }
Person.prototype.say = function(){
  console.log('hello, my name is ' + this.name);
};
function Man(name, age) {
  Person.apply(this, arguments);
}
Man.prototype = Object.create(Person.prototype);//a.
Man.prototype.constructor = Man;//b.
var man1 = new Man('pursue');
var man2 = new Man('joe');
console.log(man1.say == man2.say);
console.log(man1.name == man2.name);

In fact, the difference between parasitic combination inheritance and the above combination inheritance is only in the way of constructing the prototype object of the subclass (a. and b.). The Object.creat(obj) method is used here, which will process the incoming obj The object is shallow copied, similar to:

function create(obj){
  function T(){};
  T.prototype = obj;
  return new T();
}

Therefore, a. will connect the prototype object of the subclass with the prototype object of the parent class well, instead of directly copying the prototype of the subclass like the general combined inheritance (such as Man.prototype = new Person();), this is just a very violent way to overwrite attributes. The parasitic combination inheritance method inherits instance attributes and prototype attributes separately, which is more reasonable in implementation.

Note: Code b. will not change the result of instanceof, but it is more rigorous for scenarios where construcor is needed.

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