ES6中引入class语法糖来定义类,采用extends关键字来实现类的继承。相比于ES5中构造函数的方式在书写的简便性上有了质的提升。但是万变不离其宗,本文带你解密语法糖背后的实现原理,看完你会对class和extends有更深入的理解和认识
类中由四种方法构成:
class Person {
// 1 构造方法
// 对应ES5中来调用new构造函数后,执行的5步操作
constructor(name, age) {
this.name = name;
this.age = age;
this._address = "上海市";
}
// 2 实例方法
eat() {
console.log(this.name + ' is eating');
}
// 3 访问器方法
get address() {
console.log("拦截访问操作");
return this._address;
}
set address(value) {
console.log("拦截赋值操作");
this._address = value;
}
// 4 静态方法
static randomPerson() {
const names = ["sd", "fr", "re", "weee"];
const nameIdx = Math.floor(Math.random() * names.length);
const name = names[nameIdx];
const age = Math.floor(Math.random() * 100);
return new Person(name, age);
}
}
创建实例对象
// 创建对象实例
const p = new Person('zs', 18);
console.log(p); // {name: 'zs', age: 18, _address: '上海市'};
// 调用实例方法
p.eat() // 'zs is eating'
// 测试访问器方法
// 注意address属性挂载到是Person原型对象上的
console.log(p.address) // '拦截访问操作'
p.address = '北京市'; // '拦截赋值操作'
// 测试静态方法
// 注意:静态方法的调用方式
const p2 = Person.randomPerson()
类继承涉及到关键字extends
,下面演示创建子类Student继承父类Person,涉及到两个关键字extends和super
class Student extends Person {
// 构造方法
constructor(name, age, id) {
// JS引擎在解析子类的时候要求:如果使用了继承extends,那么子类的构造方法中,使用this之前必须调用父类的构造方法
super(name, age);
this.id = id;
}
// 实例方法
// 和父类方法同名,属于方法重写
eat() {
// 借助super关键字来调用父类方法
super.eat();
console.log('Student' + this.name + ' is eating');
}
// 访问器方法 同样可以定义,不再赘述
// 静态方法
static staticMethod() {
super.randomPerson(); // 只有在子类静态方法中才可以调用父类的静态方法,子类其他方法不可
console.log("student static method");
}
}
如果还想了解class和extends等语法糖下包裹的内核,提升自己的JS功底,增加对于类的理解,请沉下心继续看。
class是ES6中提出的构造类的语法糖,一些低版本的浏览器不支持ES6语法,所以需要借助于babel等工具将其转译为ES5的构造函数。babel的在线转化平台:https://babeljs.io/
"use strict";
// 函数若没有作为构造函数调用,直接抛出错误
function _classCallCheck(instance, Constructor) {
if (!(instance instanceof Constructor)) {
throw new TypeError("Cannot call a class as a function");
}
}
// 批量设置对象属性描述符
function _defineProperties(target, props) {
for (var i = 0; i < props.length; i++) {
var descriptor = props[i];
descriptor.enumerable = descriptor.enumerable || false;
descriptor.configurable = true;
if ("value" in descriptor) descriptor.writable = true;
Object.defineProperty(target, descriptor.key, descriptor);
}
}
// ⭐为构造函数挂载方法
function _createClass(Constructor, protoProps, staticProps) {
// 将实例方法、访问器属性挂载到构造函数Person的原型对象上
if (protoProps) _defineProperties(Constructor.prototype, protoProps);
// 将静态方法添加到构造函数Person对象上
if (staticProps) _defineProperties(Constructor, staticProps);
// 设置构造函数Person的原型对象不可更改
Object.defineProperty(Constructor, "prototype", { writable: false });
return Constructor;
}
// ⭐从这里读
var Person = /*#__PURE__*/ (function () {
// 1 构造方法
// 对应ES5中来调用new构造函数后,执行的5步操作
function Person(name, age) {
_classCallCheck(this, Person); // 检查Person是否会作为构造函数来调用
this.name = name;
this.age = age;
this._address = "上海市";
}
_createClass(
Person,
[
{
key: "eat",
value: function eat() {
console.log(this.name + " is eating");
} // 2 实例方法
},
{
key: "address",
get: function get() {
console.log("拦截访问操作");
return this._address;
},
set: function set(value) {
console.log("拦截赋值操作");
this._address = value;
} // 3 访问器方法
}
],
[
{
key: "randomPerson",
value: function randomPerson() {
var names = ["sd", "fr", "re", "weee"];
var nameIdx = Math.floor(Math.random() * names.length);
var name = names[nameIdx];
var age = Math.floor(Math.random() * 100);
return new Person(name, age);
}// 4 静态方法
}
]
);
return Person;
})();
tips
/*#__PURE__*/
标记的好处:"use strict";
function _typeof(obj) {
"@babel/helpers - typeof";
return (
(_typeof =
"function" == typeof Symbol && "symbol" == typeof Symbol.iterator
? function (obj) {
return typeof obj;
}
: function (obj) {
return obj &&
"function" == typeof Symbol &&
obj.constructor === Symbol &&
obj !== Symbol.prototype
? "symbol"
: typeof obj;
}),
_typeof(obj)
);
}
function _classCallCheck(instance, Constructor) {
if (!(instance instanceof Constructor)) {
throw new TypeError("Cannot call a class as a function");
}
}
function _defineProperties(target, props) {
for (var i = 0; i < props.length; i++) {
var descriptor = props[i];
descriptor.enumerable = descriptor.enumerable || false;
descriptor.configurable = true;
if ("value" in descriptor) descriptor.writable = true;
Object.defineProperty(target, descriptor.key, descriptor);
}
}
function _createClass(Constructor, protoProps, staticProps) {
if (protoProps) _defineProperties(Constructor.prototype, protoProps);
if (staticProps) _defineProperties(Constructor, staticProps);
Object.defineProperty(Constructor, "prototype", { writable: false });
return Constructor;
}
function _get() {
if (typeof Reflect !== "undefined" && Reflect.get) {
_get = Reflect.get;
} else {
_get = function _get(target, property, receiver) {
var base = _superPropBase(target, property);
if (!base) return;
var desc = Object.getOwnPropertyDescriptor(base, property);
if (desc.get) {
return desc.get.call(arguments.length < 3 ? target : receiver);
}
return desc.value;
};
}
return _get.apply(this, arguments);
}
function _superPropBase(object, property) {
while (!Object.prototype.hasOwnProperty.call(object, property)) {
object = _getPrototypeOf(object);
if (object === null) break;
}
return object;
}
// subClass: Student
// superClass: Parent
function _inherits(subClass, superClass) {
if (typeof superClass !== "function" && superClass !== null) {
throw new TypeError("Super expression must either be null or a function");
}
subClass.prototype = Object.create(superClass && superClass.prototype, {
constructor: { value: subClass, writable: true, configurable: true }
});
Object.defineProperty(subClass, "prototype", { writable: false });
if (superClass) _setPrototypeOf(subClass, superClass);
}
// Student.__proto__ = Parent
function _setPrototypeOf(o, p) {
_setPrototypeOf =
Object.setPrototypeOf ||
function _setPrototypeOf(o, p) {
o.__proto__ = p;
return o;
};
return _setPrototypeOf(o, p);
}
function _createSuper(Derived) {
var hasNativeReflectConstruct = _isNativeReflectConstruct();
return function _createSuperInternal() {
var Super = _getPrototypeOf(Derived),
result;
if (hasNativeReflectConstruct) {
var NewTarget = _getPrototypeOf(this).constructor;
result = Reflect.construct(Super, arguments, NewTarget);
} else {
result = Super.apply(this, arguments);
}
return _possibleConstructorReturn(this, result);
};
}
function _possibleConstructorReturn(self, call) {
if (call && (_typeof(call) === "object" || typeof call === "function")) {
return call;
} else if (call !== void 0) {
throw new TypeError(
"Derived constructors may only return object or undefined"
);
}
return _assertThisInitialized(self);
}
function _assertThisInitialized(self) {
if (self === void 0) {
throw new ReferenceError(
"this hasn't been initialised - super() hasn't been called"
);
}
return self;
}
function _isNativeReflectConstruct() {
if (typeof Reflect === "undefined" || !Reflect.construct) return false;
if (Reflect.construct.sham) return false;
if (typeof Proxy === "function") return true;
try {
Boolean.prototype.valueOf.call(
Reflect.construct(Boolean, [], function () {})
);
return true;
} catch (e) {
return false;
}
}
function _getPrototypeOf(o) {
_getPrototypeOf = Object.setPrototypeOf
? Object.getPrototypeOf
: function _getPrototypeOf(o) {
return o.__proto__ || Object.getPrototypeOf(o);
};
return _getPrototypeOf(o);
}
// ⭐重要
var Student = /*#__PURE__*/ (function (_Person) {
_inherits(Student, _Person); // 实现继承
var _super = _createSuper(Student); // Person类不能直接调用,所以要创建一个
// 构造方法
function Student(name, age, id) {
var _this;
_classCallCheck(this, Student);
// JS引擎在解析子类的时候要求:如果使用了继承extends,那么子类的构造方法中,使用this之前必须调用父类的构造方法
_this = _super.call(this, name, age);
_this.id = id;
return _this;
} // 实例方法
// 和父类方法同名,属于方法重写
_createClass(
Student,
[
{
key: "eat",
value: function eat() {
// 借助super关键字来调用父类方法
_get(_getPrototypeOf(Student.prototype), "eat", this).call(this);
console.log("Student" + this.name + " is eating");
} // 访问器方法 同样可以定义,不再赘述
// 静态方法
}
],
[
{
key: "staticMethod",
value: function staticMethod() {
_get(_getPrototypeOf(Student), "randomPerson", this).call(this); // 只有在子类静态方法中才可以调用父类的静态方法,子类其他方法不可
console.log("student static method");
}
}
]
);
return Student;
})(Person);