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本篇内容主要讲解“无constructor的class类还能new吗”，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习“无constructor的class类还能new吗”吧!
某一天晚上跟"混子瑶"聊天，下面模拟一下对话情景。
我：混子瑶，今天学了什么啊？
混子瑶：今天学习了TypeScript的class类高级类型啊！(然后发了一张截图...，图上有明显的两个类 )
class Point { x: number; y: number };
class Point2D ( x: number; y: number };
const p: Point = new Point2D();我：Point2D都没有constructor，它能被new吗?
混子瑶：可以啊，你看它又没报错：
我：这么神奇的嘛？
混子瑶： emmmm...
果真是这么神奇的嘛？来试一下不就知道咯！
class是ES6提供的一个语法糖，本质是一个函数，它具有constructor，static默认方法... 咦？既然constructor是class类默认的，那岂不是显示，隐式都会默认去调用吗？文章的标题不就是一个子虚乌有的吗？ 我们上babel来进行一下语法降级。
class A {
x = 1;
y = 2
}
const a = new A(10,20)
console.log(a); // {x: 1, y: 2}
class B {
constructor(x, y){
this.x = x;
this.y = y;
}
x = 1;
y = 2;
}
const b = new B(30,40)
console.log(b); // {x: 30, y: 40}
// 降级之后的代码
"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 _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, _toPropertyKey(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 _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
function _defineProperty(obj, key, value) { key = _toPropertyKey(key); if (key in obj) { Object.defineProperty(obj, key, { value: value, enumerable: true, configurable: true, writable: true }); } else { obj[key] = value; } return obj; }
function _toPropertyKey(arg) { var key = _toPrimitive(arg, "string"); return _typeof(key) === "symbol" ? key : String(key); }
function _toPrimitive(input, hint) { if (_typeof(input) !== "object" || input === null) return input; var prim = input[Symbol.toPrimitive]; if (prim !== undefined) { var res = prim.call(input, hint || "default"); if (_typeof(res) !== "object") return res; throw new TypeError("@@toPrimitive must return a primitive value."); } return (hint === "string" ? String : Number)(input); }
var A = /*#__PURE__*/_createClass(function A() {
_classCallCheck(this, A);
_defineProperty(this, "x", 1);
_defineProperty(this, "y", 2);
});
var a = new A(10, 20);
console.log(a); // {x: 1, y: 2}
var B = /*#__PURE__*/_createClass(function B(x, y) {
_classCallCheck(this, B);
_defineProperty(this, "x", 1);
_defineProperty(this, "y", 2);
this.x = x;
this.y = y;
});
var b = new B(30, 40);
console.log(b); // {x: 30, y: 40}利用babel降级把ES6转化成了ES5代码，更能证明class只是一个语法糖，本质只是一个函数。那我们来研究一下这些函数吧！????????????????
var B = /*#__PURE__*/ _createClass(function B(x, y) {
_classCallCheck(this, B)
_defineProperty(this, 'x', 1)
_defineProperty(this, 'y', 2)
this.x = x // 这里的this为new出来的 B{}
this.y = y // 这里的this为new出来的 B{}
})
var b = new B(30, 40)
console.log(b) // {x: 30, y: 40}function _createClass(Constructor, protoProps, staticProps) {
// 其中Constructor为f B（x，y），创造的一个ES5构造函数
// protoProps :undefined 属性
// staticProps: undefined 静态属性
if (protoProps) _defineProperties(Constructor.prototype, protoProps)
if (staticProps) _defineProperties(Constructor, staticProps)
Object.defineProperty(Constructor, 'prototype', { writable: false })
return Constructor // 返回f B（x，y）
}function _classCallCheck(instance, Constructor) {
// instance = B{} 由new创造出来
// Constructor = f B(x, y)
if (!(instance instanceof Constructor)) {
throw new TypeError('Cannot call a class as a function')
}
}function _defineProperty(obj, key, value) {
// obj = B{}
// key = "x"
// value = 1
key = _toPropertyKey(key) // 取到原始值key
if (key in obj) { // 如果key是实例属性或者在原型链上，就做劫持
Object.defineProperty(obj, key, {
value: value,
enumerable: true,
configurable: true,
writable: true
})
} else { // 不在的话，就添加key
obj[key] = value
}
return obj // 返回实例对象
}function _toPropertyKey(arg) {
// arg = "x"
var key = _toPrimitive(arg, 'string') // "x"
return _typeof(key) === 'symbol' ? key : String(key)
}function _toPrimitive(input, hint) {
// input = "x"
// hint = "string"
// 如果input是原始值，并且不等于null，则直接返回
if (_typeof(input) !== 'object' || input === null) return input
var prim = input[Symbol.toPrimitive] // 如果是对象，则需要拆箱得到原始值
if (prim !== undefined) { // 如果是不等于undefined，则表示有原始值
var res = prim.call(input, hint || 'default')
if (_typeof(res) !== 'object') return res
throw new TypeError('@@toPrimitive must return a primitive value.')
}
return (hint === 'string' ? String : Number)(input) // 如果是undefined则表示，没有取到原始值，需要继续拆箱调用Sting("x")取到原始值
}function _typeof(obj) {
// obj = "x"
'@babel/helpers - typeof' // babel提供的解析模块
return (
(_typeof =
// 验证是否支持Symbol
'function' == typeof Symbol && 'symbol' == typeof Symbol.iterator
? function (obj) {
return typeof obj // "string"
} // 支持Symbol，则_typeof = fucntion(obj){return typeof obj}
: function (obj) { // 不支持Symbol的话，则会判断当前的单一职责，保证一个实例的情况
return obj &&
'function' == typeof Symbol &&
obj.constructor === Symbol &&
obj !== Symbol.prototype
? 'symbol'
: typeof obj // "string"
}),
_typeof(obj)
)
}function _defineProperties(target, props) {
// target = Constructor.prototype
// props = props属性
for (var i = 0; i < props.length; i++) {
var descriptor = props[i] // 遍历属性
descriptor.enumerable = descriptor.enumerable || false // 设置不可枚举 静态属性无法被实例化
descriptor.configurable = true // 可扩展
if ('value' in descriptor) descriptor.writable = true // 科协
Object.defineProperty(target, _toPropertyKey(descriptor.key), descriptor)
}
}静态属性无法被实例化
我们有如下代码
// class B
class B {
static q = 1;
m = 3;
constructor(x, y){
this.x = x;
this.y = y;
}
}
// class E
class E extends B {
constructor(x,y){
super(x,y)
this.x = x;
this.y =y
}
m = 10;
n = 20;
}
const e = new E(100,200)
const b = new B(30,40)
console.log(e);
console.log(b);执行结果：
代码降级：
"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 _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); }
function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf ? Object.setPrototypeOf.bind() : 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.bind() : function _getPrototypeOf(o) { return o.__proto__ || Object.getPrototypeOf(o); }; return _getPrototypeOf(o); }
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, _toPropertyKey(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 _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
function _defineProperty(obj, key, value) { key = _toPropertyKey(key); if (key in obj) { Object.defineProperty(obj, key, { value: value, enumerable: true, configurable: true, writable: true }); } else { obj[key] = value; } return obj; }
function _toPropertyKey(arg) { var key = _toPrimitive(arg, "string"); return _typeof(key) === "symbol" ? key : String(key); }
function _toPrimitive(input, hint) { if (_typeof(input) !== "object" || input === null) return input; var prim = input[Symbol.toPrimitive]; if (prim !== undefined) { var res = prim.call(input, hint || "default"); if (_typeof(res) !== "object") return res; throw new TypeError("@@toPrimitive must return a primitive value."); } return (hint === "string" ? String : Number)(input); }
var B = /*#__PURE__*/_createClass(function B(x, y) {
_classCallCheck(this, B);
_defineProperty(this, "m", 3);
this.x = x;
this.y = y;
});
_defineProperty(B, "q", 1);
var b = new B(30, 40);
console.log(b);
var E = /*#__PURE__*/function (_B) {
_inherits(E, _B); // _B为f B（x,y） E 为f E(x, y)
var _super = _createSuper(E);
function E(x, y) {
var _this;
_classCallCheck(this, E);
_this = _super.call(this, x, y);
_defineProperty(_assertThisInitialized(_this), "m", 10);
_defineProperty(_assertThisInitialized(_this), "n", 20);
_this.x = x;
_this.y = y;
return _this;
}
return _createClass(E);
}(B);
var e = new E(100, 200);
console.log(e);在这里我们看到了super关键字被_inherits、_createSuper代替，extends关键字也被函数替代，那么我们来研究一下这个代码。
function _inherits(subClass, superClass) { // subClass为子类的构造函数，superClass为父类的构造函数
if (typeof superClass !== 'function' && superClass !== null) {
throw new TypeError('Super expression must either be null or a function')
}
// 创建子类的原型
subClass.prototype = Object.create(superClass && superClass.prototype, {
constructor: { value: subClass, writable: true, configurable: true }
})
Object.defineProperty(subClass, 'prototype', { writable: false })
// 绑定子类原型
if (superClass) _setPrototypeOf(subClass, superClass)
}function _setPrototypeOf(o, p) { // o为子类的构造函数，p为父类的构造函数
_setPrototypeOf = Object.setPrototypeOf
? Object.setPrototypeOf.bind()
: function _setPrototypeOf(o, p) {
o.__proto__ = p
return o
}
return _setPrototypeOf(o, p) // 绑定原型
}function _createSuper(Derived) { // Derived = f E(x,y)
// 校验能不能用Reflect
var hasNativeReflectConstruct = _isNativeReflectConstruct()
return function _createSuperInternal() {
var Super = _getPrototypeOf(Derived), // 获得原型对象
result // 创建实例结果
if (hasNativeReflectConstruct) { // 如果能用Reflect，则调用Reflect.construct来创建实例对象
var NewTarget = _getPrototypeOf(this).constructor
result = Reflect.construct(Super, arguments, NewTarget)
} else {
result = Super.apply(this, arguments) // 不能则把父类当做普通函数执行，this改变为子类实例对象
}
return _possibleConstructorReturn(this, result)
}
}function _isNativeReflectConstruct() {
// 判断是否可用Reflect， 不可被new，因为没有construct
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
}
}// 获取原型对象
function _getPrototypeOf(o) {
_getPrototypeOf = Object.setPrototypeOf
? Object.getPrototypeOf.bind()
: function _getPrototypeOf(o) {
return o.__proto__ || Object.getPrototypeOf(o)
}
return _getPrototypeOf(o)
}_possibleConstructorReturn、_assertThisInitialized则是检验子类constructor规则与实例存在与否。 之后便是走_createclass那一套逻辑，所以这就是class继承相关的东西，跟ES5中的组合寄生继承实现的很类似。
到此，相信大家对“无constructor的class类还能new吗”有了更深的了解，不妨来实际操作一番吧！这里是亿速云网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习！
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