js中的类和constructor

问题
一直搞不清constructor和super是干什么用的。

前提
了解js的继承

原型继承
原型继承是通过利用js的原型链

function People(name, age) {
  this.name = name;
  this.age = age;
}
People.prototype.sayName = function() {
  console.log('my name is ' + this.name);
}

function Man() {}
Man.prototype = new People('小明', 20);

let m = new Man();

m.sayName();

call或者apply实现继承
利用call和apply可以绑定this

function People(name, age) {
  this.name = name;
  this.age = age;
}
People.prototype.sayName = function() {
  console.log('my name is ' + this.name);
}

function Man() {
  People.call(this, 20);
}

let m = new Man();

m.sayName();

js的类
其实js的类就是语法糖

class Polygon {
  constructor(height, width) {
    this.name = 'Polygon';
    this.height = height;
    this.width = width;
  }
  sayName() {
    console.log('Hi, I am a ', this.name + '.');
  }
}

class中的构造函数其实就相当于以前的这种写法

function Polygon(height, width) {
  this.name = 'Polygon';
  this.height = height;
  this.width = width;
  this.sayName = sayName;
}

function sayName() {
  console.log('Hi, I am a ', this.name + '.');
}

只是不一样的写法，super是调用父类构造函数，参数就是传到父类构造函数的参数

class Polygon {
  constructor(height, width) {
    this.name = 'Polygon';
    this.height = height;
    this.width = width;
  }
  sayName() {
    console.log('Hi, I am a ', this.name + '.');
  }
}

class Square extends Polygon {
  constructor(length) {
    this.height; 
    // ReferenceError，super 需要先被调用！
    
/*
   这里，它调用父类的构造函数的 length, 
   作为Polygon 的 width和 height.
*/ 
    super(length, length);
    
/*
    注意: 在派生的类中, 在你可以使用'this'之前, 必须先调用super()。
    忽略这, 这将导致引用错误。
*/
    this.name = 'Square';
  }

  get area() {
    return this.height * this.width;
  }

  set area(value) {
    this.area = value;
  } 
}