無constructor的class類還能new嗎問題解析

更新時間：2023年03月07日 14:19:13 作者：一溪之石

這篇文章主要為大家介紹了無constructor的class類是否還能new的問題解析，有需要的朋友可以借鑒參考下，希望能夠有所幫助，祝大家多多進步，早日升職加薪

前言

某一天晚上跟"混子瑤"聊天，下面模擬一下對話情景。

我：混子瑤，今天學(xué)了什么?。?/p>

混子瑤：今天學(xué)習(xí)了TypeScript的class類高級類型?。?然后發(fā)了一張截圖...，圖上有明顯的兩個類 )

 class Point { x: number; y: number };
 class Point2D ( x: number; y: number };
 const p: Point = new Point2D();

我：Point2D都沒有constructor，它能被new嗎?

混子瑤：可以啊，你看它又沒報錯：

我：這么神奇的嘛？

混子瑤： emmmm...

果真是這么神奇的嘛？來試一下不就知道咯！

class語法糖

class是ES6提供的一個語法糖，本質(zhì)是一個函數(shù)，它具有constructor，static默認方法... 咦？既然constructor是class類默認的，那豈不是顯示，隱式都會默認去調(diào)用嗎？文章的標(biāo)題不就是一個子虛烏有的嗎？我們上babel來進行一下語法降級。

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}
// 降級之后的代碼
"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降級把ES6轉(zhuǎn)化成了ES5代碼，更能證明class只是一個語法糖，本質(zhì)只是一個函數(shù)。那我們來研究一下這些函數(shù)吧！????????

var B = /*#__PURE__*/ _createClass(function B(x, y) {
  _classCallCheck(this, B)
  _defineProperty(this, 'x', 1)
  _defineProperty(this, 'y', 2)
  this.x = x // 這里的this為new出來的 B{}
  this.y = y // 這里的this為new出來的 B{}
})
var b = new B(30, 40)
console.log(b) // {x: 30, y: 40}

_createClass

function _createClass(Constructor, protoProps, staticProps) {
  // 其中Constructor為f B（x，y），創(chuàng)造的一個ES5構(gòu)造函數(shù)
  // protoProps :undefined 屬性
  // staticProps: undefined 靜態(tài)屬性
  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）
}

_classCallCheck

function _classCallCheck(instance, Constructor) {
  // instance = B{} 由new創(chuàng)造出來
  // Constructor = f B(x, y)
  if (!(instance instanceof Constructor)) {
    throw new TypeError('Cannot call a class as a function')
  }
}

_defineProperty

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是實例屬性或者在原型鏈上，就做劫持
    Object.defineProperty(obj, key, {
      value: value,
      enumerable: true,
      configurable: true,
      writable: true
    })
  } else { // 不在的話，就添加key
    obj[key] = value
  }
  return obj // 返回實例對象
}

_toPropertyKey

function _toPropertyKey(arg) {
  // arg = "x"
  var key = _toPrimitive(arg, 'string') // "x"
  return _typeof(key) === 'symbol' ? key : String(key)
}

_toPrimitive

function _toPrimitive(input, hint) {
  // input = "x"
  // hint = "string"
  // 如果input是原始值，并且不等于null，則直接返回
  if (_typeof(input) !== 'object' || input === null) return input
  var prim = input[Symbol.toPrimitive] // 如果是對象，則需要拆箱得到原始值
  if (prim !== undefined) { // 如果是不等于undefined，則表示有原始值
    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則表示，沒有取到原始值，需要繼續(xù)拆箱調(diào)用Sting("x")取到原始值
}

_typeof

function _typeof(obj) {
  // obj = "x"
  '@babel/helpers - typeof' // babel提供的解析模塊
  return (
    (_typeof =
      // 驗證是否支持Symbol
      'function' == typeof Symbol && 'symbol' == typeof Symbol.iterator
        ? function (obj) {
            return typeof obj // "string"
          } // 支持Symbol，則_typeof = fucntion(obj){return typeof obj}
        : function (obj) { // 不支持Symbol的話，則會判斷當(dāng)前的單一職責(zé)，保證一個實例的情況
            return obj && 
              'function' == typeof Symbol &&
              obj.constructor === Symbol &&
              obj !== Symbol.prototype
              ? 'symbol'
              : typeof obj // "string"
          }),
    _typeof(obj)
  )
}

_defineProperties

function _defineProperties(target, props) {
  // target = Constructor.prototype
  // props = props屬性
  for (var i = 0; i < props.length; i++) {
    var descriptor = props[i] // 遍歷屬性
    descriptor.enumerable = descriptor.enumerable || false // 設(shè)置不可枚舉 靜態(tài)屬性無法被實例化
    descriptor.configurable = true // 可擴展
    if ('value' in descriptor) descriptor.writable = true // 科協(xié)
    Object.defineProperty(target, _toPropertyKey(descriptor.key), descriptor)
  }
}

靜態(tài)屬性無法被實例化

關(guān)于class的繼承

我們有如下代碼

// 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);

執(zhí)行結(jié)果：

代碼降級：

"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為f B（x,y） E 為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);

在這里我們看到了super關(guān)鍵字被_inherits、_createSuper代替，extends關(guān)鍵字也被函數(shù)替代，那么我們來研究一下這個代碼。

_inherits

function _inherits(subClass, superClass) { // subClass為子類的構(gòu)造函數(shù)，superClass為父類的構(gòu)造函數(shù)
  if (typeof superClass !== 'function' && superClass !== null) {
    throw new TypeError('Super expression must either be null or a function')
  }
  // 創(chuàng)建子類的原型
  subClass.prototype = Object.create(superClass && superClass.prototype, {
    constructor: { value: subClass, writable: true, configurable: true }
  })
  Object.defineProperty(subClass, 'prototype', { writable: false })
  // 綁定子類原型
  if (superClass) _setPrototypeOf(subClass, superClass)
}

_setPrototypeOf

function _setPrototypeOf(o, p) { // o為子類的構(gòu)造函數(shù)，p為父類的構(gòu)造函數(shù)
  _setPrototypeOf = Object.setPrototypeOf
    ? Object.setPrototypeOf.bind()
    : function _setPrototypeOf(o, p) {
        o.__proto__ = p
        return o
      }
  return _setPrototypeOf(o, p) // 綁定原型
}

_createSuper

function _createSuper(Derived) { // Derived = f E(x,y)
  // 校驗?zāi)懿荒苡肦eflect
  var hasNativeReflectConstruct = _isNativeReflectConstruct()
  return function _createSuperInternal() {
    var Super = _getPrototypeOf(Derived), // 獲得原型對象
      result // 創(chuàng)建實例結(jié)果
    if (hasNativeReflectConstruct) { // 如果能用Reflect，則調(diào)用Reflect.construct來創(chuàng)建實例對象
      var NewTarget = _getPrototypeOf(this).constructor
      result = Reflect.construct(Super, arguments, NewTarget)
    } else {
      result = Super.apply(this, arguments) // 不能則把父類當(dāng)做普通函數(shù)執(zhí)行，this改變?yōu)樽宇悓嵗龑ο?
    }
    return _possibleConstructorReturn(this, result)
  }
}

_isNativeReflectConstruct

function _isNativeReflectConstruct() {
  // 判斷是否可用Reflect， 不可被new，因為沒有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
  }
}

_getPrototypeOf

// 獲取原型對象
function _getPrototypeOf(o) {
  _getPrototypeOf = Object.setPrototypeOf
    ? Object.getPrototypeOf.bind()
    : function _getPrototypeOf(o) {
        return o.__proto__ || Object.getPrototypeOf(o)
      }
  return _getPrototypeOf(o)
}

_possibleConstructorReturn、_assertThisInitialized則是檢驗子類constructor規(guī)則與實例存在與否。之后便是走_createclass那一套邏輯，所以這就是class繼承相關(guān)的東西，跟ES5中的組合寄生繼承實現(xiàn)的很類似。