js如何实现继承

原型链继承

通过让子类的原型对象指向父类的实例来实现继承。子类实例能够访问父类的属性和方法。

function Parent() {
  this.name = 'parent';
}
Parent.prototype.sayName = function() {
  console.log(this.name);
};

function Child() {}
Child.prototype = new Parent();

const child = new Child();
child.sayName(); // 'parent'

构造函数继承

在子类构造函数中调用父类构造函数，使用call或apply方法改变this指向。

function Parent(name) {
  this.name = name;
}
function Child(name) {
  Parent.call(this, name);
}

const child = new Child('child');
console.log(child.name); // 'child'

组合继承

结合原型链继承和构造函数继承，既继承父类实例属性又继承原型方法。

function Parent(name) {
  this.name = name;
}
Parent.prototype.sayName = function() {
  console.log(this.name);
};

function Child(name) {
  Parent.call(this, name);
}
Child.prototype = new Parent();
Child.prototype.constructor = Child;

const child = new Child('child');
child.sayName(); // 'child'

原型式继承

使用Object.create()方法基于现有对象创建新对象。

const parent = {
  name: 'parent',
  sayName: function() {
    console.log(this.name);
  }
};

const child = Object.create(parent);
child.name = 'child';
child.sayName(); // 'child'

寄生式继承

在原型式继承的基础上增强对象，添加额外方法。

function createChild(parent) {
  const child = Object.create(parent);
  child.sayHi = function() {
    console.log('hi');
  };
  return child;
}

const parent = { name: 'parent' };
const child = createChild(parent);
child.sayHi(); // 'hi'

寄生组合式继承

目前最理想的继承方式，避免组合继承中两次调用父类构造函数的问题。

function inheritPrototype(child, parent) {
  const prototype = Object.create(parent.prototype);
  prototype.constructor = child;
  child.prototype = prototype;
}

function Parent(name) {
  this.name = name;
}
Parent.prototype.sayName = function() {
  console.log(this.name);
};

function Child(name) {
  Parent.call(this, name);
}
inheritPrototype(Child, Parent);

const child = new Child('child');
child.sayName(); // 'child'

ES6 class继承

使用extends关键字实现继承，语法更简洁直观。

class Parent {
  constructor(name) {
    this.name = name;
  }
  sayName() {
    console.log(this.name);
  }
}

class Child extends Parent {
  constructor(name) {
    super(name);
  }
}

const child = new Child('child');
child.sayName(); // 'child'