对js 中的类一些感悟

李赣嘛
54 阅读6分钟 
// es5创建类的工厂函数写法
function range(from, to) {
    let r = Object.create(range.methods);
    r.from = from;
    r.to = to;
    return r;
}

range.methods = {
    includes(v) {
        return v >= this.from && v <= this.to;
    },


    toString() {
        return "(" + this.from + "..." + this.to + ")"
    },

    *[Symbol.iterator]() {
        for (let x = Math.ceil(this.from); x <= this.to; x++) yield x;
    }
}
let r = range(1, 3)

// es5写法
function Range(from, to) {
    // let this = Object.create(range.prototype);
    this.from = from;
    this.to = to;

    // return this
}

Range.prototype = {
    includes: function (v) {
        return v >= this.from && v <= this.to;
    },

    toString: function () {
        return "(" + this.from + "..." + this.to + ")"
    },

    [Symbol.iterator]: function* () {
        for (let x = Math.ceil(this.from); x <= this.to; x++) yield x;
    }
}
let n = new Range(1, 3);
简单理解new 做的事情就是 把用构造函数的prototype 作为新创建的对象的原型 然后返回这个原型

// 为了不破坏原本的原型链，在之前的原型基础上增加方法
Range.prototype.includes = function (v) {
    return v >= this.from && v <= this.to;
}

Range.prototype.toString = function () {
    return "(" + this.from + "..." + this.to + ")"
},

// es6 写法
class Range {

    constructor(from, to) {
        this.from = from;
        this.to = to;
    }

    includes(v) {
        return v >= this.from && v <= this.to;
    }
    toString() {
        return "(" + this.from + "..." + this.to + ")"
    }
    *[Symbol.iterator]() {
        for (let x = Math.ceil(this.from); x <= this.to; x++) yield x;
    }
}
    
let n = new Range(1, 3);

上述用es6的写法用babel转为es5
"use strict";

function _instanceof(n, e) { return null != e && "undefined" != typeof Symbol && e[Symbol.hasInstance] ? !!e[Symbol.hasInstance](n) : n instanceof e; }
function _typeof(o) { "@babel/helpers - typeof"; return _typeof = "function" == typeof Symbol && "symbol" == typeof Symbol.iterator ? function (o) { return typeof o; } : function (o) { return o && "function" == typeof Symbol && o.constructor === Symbol && o !== Symbol.prototype ? "symbol" : typeof o; }, _typeof(o); }
function _regeneratorRuntime() { "use strict"; /*! regenerator-runtime -- Copyright (c) 2014-present, Facebook, Inc. -- license (MIT): https://github.com/facebook/regenerator/blob/main/LICENSE */ _regeneratorRuntime = function _regeneratorRuntime() { return e; }; var t, e = {}, r = Object.prototype, n = r.hasOwnProperty, o = Object.defineProperty || function (t, e, r) { t[e] = r.value; }, i = "function" == typeof Symbol ? Symbol : {}, a = i.iterator || "@@iterator", c = i.asyncIterator || "@@asyncIterator", u = i.toStringTag || "@@toStringTag"; function define(t, e, r) { return Object.defineProperty(t, e, { value: r, enumerable: !0, configurable: !0, writable: !0 }), t[e]; } try { define({}, ""); } catch (t) { define = function define(t, e, r) { return t[e] = r; }; } function wrap(t, e, r, n) { var i = e && _instanceof(e.prototype, Generator) ? e : Generator, a = Object.create(i.prototype), c = new Context(n || []); return o(a, "_invoke", { value: makeInvokeMethod(t, r, c) }), a; } function tryCatch(t, e, r) { try { return { type: "normal", arg: t.call(e, r) }; } catch (t) { return { type: "throw", arg: t }; } } e.wrap = wrap; var h = "suspendedStart", l = "suspendedYield", f = "executing", s = "completed", y = {}; function Generator() {} function GeneratorFunction() {} function GeneratorFunctionPrototype() {} var p = {}; define(p, a, function () { return this; }); var d = Object.getPrototypeOf, v = d && d(d(values([]))); v && v !== r && n.call(v, a) && (p = v); var g = GeneratorFunctionPrototype.prototype = Generator.prototype = Object.create(p); function defineIteratorMethods(t) { ["next", "throw", "return"].forEach(function (e) { define(t, e, function (t) { return this._invoke(e, t); }); }); } function AsyncIterator(t, e) { function invoke(r, o, i, a) { var c = tryCatch(t[r], t, o); if ("throw" !== c.type) { var u = c.arg, h = u.value; return h && "object" == _typeof(h) && n.call(h, "__await") ? e.resolve(h.__await).then(function (t) { invoke("next", t, i, a); }, function (t) { invoke("throw", t, i, a); }) : e.resolve(h).then(function (t) { u.value = t, i(u); }, function (t) { return invoke("throw", t, i, a); }); } a(c.arg); } var r; o(this, "_invoke", { value: function value(t, n) { function callInvokeWithMethodAndArg() { return new e(function (e, r) { invoke(t, n, e, r); }); } return r = r ? r.then(callInvokeWithMethodAndArg, callInvokeWithMethodAndArg) : callInvokeWithMethodAndArg(); } }); } function makeInvokeMethod(e, r, n) { var o = h; return function (i, a) { if (o === f) throw Error("Generator is already running"); if (o === s) { if ("throw" === i) throw a; return { value: t, done: !0 }; } for (n.method = i, n.arg = a;;) { var c = n.delegate; if (c) { var u = maybeInvokeDelegate(c, n); if (u) { if (u === y) continue; return u; } } if ("next" === n.method) n.sent = n._sent = n.arg;else if ("throw" === n.method) { if (o === h) throw o = s, n.arg; n.dispatchException(n.arg); } else "return" === n.method && n.abrupt("return", n.arg); o = f; var p = tryCatch(e, r, n); if ("normal" === p.type) { if (o = n.done ? s : l, p.arg === y) continue; return { value: p.arg, done: n.done }; } "throw" === p.type && (o = s, n.method = "throw", n.arg = p.arg); } }; } function maybeInvokeDelegate(e, r) { var n = r.method, o = e.iterator[n]; if (o === t) return r.delegate = null, "throw" === n && e.iterator.return && (r.method = "return", r.arg = t, maybeInvokeDelegate(e, r), "throw" === r.method) || "return" !== n && (r.method = "throw", r.arg = new TypeError("The iterator does not provide a '" + n + "' method")), y; var i = tryCatch(o, e.iterator, r.arg); if ("throw" === i.type) return r.method = "throw", r.arg = i.arg, r.delegate = null, y; var a = i.arg; return a ? a.done ? (r[e.resultName] = a.value, r.next = e.nextLoc, "return" !== r.method && (r.method = "next", r.arg = t), r.delegate = null, y) : a : (r.method = "throw", r.arg = new TypeError("iterator result is not an object"), r.delegate = null, y); } function pushTryEntry(t) { var e = { tryLoc: t[0] }; 1 in t && (e.catchLoc = t[1]), 2 in t && (e.finallyLoc = t[2], e.afterLoc = t[3]), this.tryEntries.push(e); } function resetTryEntry(t) { var e = t.completion || {}; e.type = "normal", delete e.arg, t.completion = e; } function Context(t) { this.tryEntries = [{ tryLoc: "root" }], t.forEach(pushTryEntry, this), this.reset(!0); } function values(e) { if (e || "" === e) { var r = e[a]; if (r) return r.call(e); if ("function" == typeof e.next) return e; if (!isNaN(e.length)) { var o = -1, i = function next() { for (; ++o < e.length;) if (n.call(e, o)) return next.value = e[o], next.done = !1, next; return next.value = t, next.done = !0, next; }; return i.next = i; } } throw new TypeError(_typeof(e) + " is not iterable"); } return GeneratorFunction.prototype = GeneratorFunctionPrototype, o(g, "constructor", { value: GeneratorFunctionPrototype, configurable: !0 }), o(GeneratorFunctionPrototype, "constructor", { value: GeneratorFunction, configurable: !0 }), GeneratorFunction.displayName = define(GeneratorFunctionPrototype, u, "GeneratorFunction"), e.isGeneratorFunction = function (t) { var e = "function" == typeof t && t.constructor; return !!e && (e === GeneratorFunction || "GeneratorFunction" === (e.displayName || e.name)); }, e.mark = function (t) { return Object.setPrototypeOf ? Object.setPrototypeOf(t, GeneratorFunctionPrototype) : (t.__proto__ = GeneratorFunctionPrototype, define(t, u, "GeneratorFunction")), t.prototype = Object.create(g), t; }, e.awrap = function (t) { return { __await: t }; }, defineIteratorMethods(AsyncIterator.prototype), define(AsyncIterator.prototype, c, function () { return this; }), e.AsyncIterator = AsyncIterator, e.async = function (t, r, n, o, i) { void 0 === i && (i = Promise); var a = new AsyncIterator(wrap(t, r, n, o), i); return e.isGeneratorFunction(r) ? a : a.next().then(function (t) { return t.done ? t.value : a.next(); }); }, defineIteratorMethods(g), define(g, u, "Generator"), define(g, a, function () { return this; }), define(g, "toString", function () { return "[object Generator]"; }), e.keys = function (t) { var e = Object(t), r = []; for (var n in e) r.push(n); return r.reverse(), function next() { for (; r.length;) { var t = r.pop(); if (t in e) return next.value = t, next.done = !1, next; } return next.done = !0, next; }; }, e.values = values, Context.prototype = { constructor: Context, reset: function reset(e) { if (this.prev = 0, this.next = 0, this.sent = this._sent = t, this.done = !1, this.delegate = null, this.method = "next", this.arg = t, this.tryEntries.forEach(resetTryEntry), !e) for (var r in this) "t" === r.charAt(0) && n.call(this, r) && !isNaN(+r.slice(1)) && (this[r] = t); }, stop: function stop() { this.done = !0; var t = this.tryEntries[0].completion; if ("throw" === t.type) throw t.arg; return this.rval; }, dispatchException: function dispatchException(e) { if (this.done) throw e; var r = this; function handle(n, o) { return a.type = "throw", a.arg = e, r.next = n, o && (r.method = "next", r.arg = t), !!o; } for (var o = this.tryEntries.length - 1; o >= 0; --o) { var i = this.tryEntries[o], a = i.completion; if ("root" === i.tryLoc) return handle("end"); if (i.tryLoc <= this.prev) { var c = n.call(i, "catchLoc"), u = n.call(i, "finallyLoc"); if (c && u) { if (this.prev < i.catchLoc) return handle(i.catchLoc, !0); if (this.prev < i.finallyLoc) return handle(i.finallyLoc); } else if (c) { if (this.prev < i.catchLoc) return handle(i.catchLoc, !0); } else { if (!u) throw Error("try statement without catch or finally"); if (this.prev < i.finallyLoc) return handle(i.finallyLoc); } } } }, abrupt: function abrupt(t, e) { for (var r = this.tryEntries.length - 1; r >= 0; --r) { var o = this.tryEntries[r]; if (o.tryLoc <= this.prev && n.call(o, "finallyLoc") && this.prev < o.finallyLoc) { var i = o; break; } } i && ("break" === t || "continue" === t) && i.tryLoc <= e && e <= i.finallyLoc && (i = null); var a = i ? i.completion : {}; return a.type = t, a.arg = e, i ? (this.method = "next", this.next = i.finallyLoc, y) : this.complete(a); }, complete: function complete(t, e) { if ("throw" === t.type) throw t.arg; return "break" === t.type || "continue" === t.type ? this.next = t.arg : "return" === t.type ? (this.rval = this.arg = t.arg, this.method = "return", this.next = "end") : "normal" === t.type && e && (this.next = e), y; }, finish: function finish(t) { for (var e = this.tryEntries.length - 1; e >= 0; --e) { var r = this.tryEntries[e]; if (r.finallyLoc === t) return this.complete(r.completion, r.afterLoc), resetTryEntry(r), y; } }, catch: function _catch(t) { for (var e = this.tryEntries.length - 1; e >= 0; --e) { var r = this.tryEntries[e]; if (r.tryLoc === t) { var n = r.completion; if ("throw" === n.type) { var o = n.arg; resetTryEntry(r); } return o; } } throw Error("illegal catch attempt"); }, delegateYield: function delegateYield(e, r, n) { return this.delegate = { iterator: values(e), resultName: r, nextLoc: n }, "next" === this.method && (this.arg = t), y; } }, e; }
function _classCallCheck(a, n) { if (!_instanceof(a, n)) throw new TypeError("Cannot call a class as a function"); }
function _defineProperties(e, r) { for (var t = 0; t < r.length; t++) { var o = r[t]; o.enumerable = o.enumerable || !1, o.configurable = !0, "value" in o && (o.writable = !0), Object.defineProperty(e, _toPropertyKey(o.key), o); } }
function _createClass(e, r, t) { return r && _defineProperties(e.prototype, r), t && _defineProperties(e, t), Object.defineProperty(e, "prototype", { writable: !1 }), e; }
function _toPropertyKey(t) { var i = _toPrimitive(t, "string"); return "symbol" == _typeof(i) ? i : i + ""; }
function _toPrimitive(t, r) { if ("object" != _typeof(t) || !t) return t; var e = t[Symbol.toPrimitive]; if (void 0 !== e) { var i = e.call(t, r || "default"); if ("object" != _typeof(i)) return i; throw new TypeError("@@toPrimitive must return a primitive value."); } return ("string" === r ? String : Number)(t); }
var Range = /*#__PURE__*/function () {
  function Range(from, to) {
    _classCallCheck(this, Range);
    this.from = from;
    this.to = to;
  }
  return _createClass(Range, [{
    key: "includes",
    value: function includes(v) {
      return v >= this.from && v <= this.to;
    }
  }, {
    key: "toString",
    value: function toString() {
      return "(" + this.from + "..." + this.to + ")";
    }
  }, {
    key: Symbol.iterator,
    value: /*#__PURE__*/_regeneratorRuntime().mark(function value() {
      var x;
      return _regeneratorRuntime().wrap(function value$(_context) {
        while (1) switch (_context.prev = _context.next) {
          case 0:
            x = Math.ceil(this.from);
          case 1:
            if (!(x <= this.to)) {
              _context.next = 7;
              break;
            }
            _context.next = 4;
            return x;
          case 4:
            x++;
            _context.next = 1;
            break;
          case 7:
          case "end":
            return _context.stop();
        }
      }, value, this);
    })
  }]);
}();

简化一下

var Range = function () {
    function Range(from, to) {
        _classCallCheck(this, Range);
        this.from = from;
        this.to = to;
    }
    return _createClass(Range, [{
        key: "includes",
        value: function includes(v) {
            return v >= this.from && v <= this.to;
        }
    }]);
}();

使用了一个立即执行函数，立即执行函数中有一个Range函数闭包。_classCallCheck是检测如上是检测作用。_createClass的作用是将对应的属性挂在原型上，再返回Range函数。所以对比es5写法上没什么不同 实际上工作原理也是完全一致。

关键辅助函数

  • _classCallCheck：

    function _classCallCheck(instance, Constructor) {
  if (!(instance instanceof Constructor)) {
    throw new TypeError("Cannot call a class as a function");
  }
}

    确保类实例通过new创建，避免this指向错误。

  • _createClass：

    function _createClass(Constructor, protoProps, staticProps) {
  if (protoProps) defineProperties(Constructor.prototype, protoProps);
  if (staticProps) defineProperties(Constructor, staticProps);
  return Constructor;
}

    将方法添加到原型或构造函数（静态方法），使用Object.defineProperty设置属性特性（如不可枚举）

子类

function Range(from, to) {
    // let this = Object.create(range.prototype);
    this.from = from;
    this.to = to;

    // return this
}

Range.prototype.includes = function (v) {
    return v >= this.from && v <= this.to;
}

Range.prototype.toString = function () {
    return "(" + this.from + "..." + this.to + ")"
}

function Span(start, span) {
    if (span >= 0) {
        this.f = start;
        this.t = start + span;
    } else {
        this.t = start;
        this.f = start + span;
    }
}
//将Range的原型定义在Span原型上
Span.prototype = Object.create(Range.prototype);
// 重新定义自己的构造函数
Span.prototype.constructor = Span;
// 重新定义Span的toString方法
Span.prototype.toString = function () {
    return `(${this.from}...+${this.to - this.from})`;
}

image.png

上述继承只能继承方法 不能继承实例属性，因为属性是在实例上的，方法是写在原型链上的。

1. 静态方法/属性的继承原理

ES6 类的静态方法/属性直接定义在类的构造函数（Class 本身）上。
当使用 extends 继承时，子类会通过原型链继承父类的静态方法/属性：

class Parent {
  static parentStaticMethod() {
    console.log("Parent 的静态方法");
  }
  static parentStaticProperty = "Parent 的静态属性";
}

class Child extends Parent {}

console.log(Child.parentStaticMethod()); // 输出: "Parent 的静态方法"
console.log(Child.parentStaticProperty); // 输出: "Parent 的静态属性"

  • 底层机制：

    • 子类 Child 的 [[Prototype]]（即 __proto__）会指向父类 Parent。
    • 因此，当访问 Child.parentStaticMethod 时，引擎会沿着原型链找到 Parent 上的静态方法。
console.log(Child.__proto__ === Parent); // true

当子类通过 extends 继承父类时，实例方法和实例属性的继承依赖于两条原型链：

2. 实例方法的继承

  • 子类实例的原型链：子类实例 → Child.prototype → Parent.prototype → Object.prototype
  • 实例方法定义在类的 prototype 对象上，子类通过 Child.prototype.__proto__ 链接到父类的 Parent.prototype：
class Parent {
  parentMethod() {
    console.log("父类实例方法");
  }
}

class Child extends Parent {
  childMethod() {
    console.log("子类实例方法");
  }
}

// 实例方法的继承链
console.log(Child.prototype.__proto__ === Parent.prototype); // true

const child = new Child();
child.parentMethod(); // 输出: "父类实例方法"（通过原型链访问）

3. 实例属性的继承

  • 实例属性需通过父类构造函数初始化，子类必须在 constructor 中调用 super()：
class Parent {
  constructor() {
    this.parentProperty = "父类实例属性";
  }
}

class Child extends Parent {
  constructor() {
    super(); // 必须调用，否则报错
    this.childProperty = "子类实例属性";
  }
}

const child = new Child();
console.log(child.parentProperty); // "父类实例属性"
console.log(child.childProperty);  // "子类实例属性"

当通过 new Child() 创建子类实例时，继承父类实例属性的过程分为以下步骤：

  1. 子类构造函数必须调用 super()

    • super() 本质是调用父类构造函数（Parent.prototype.constructor）。
    • 这一步会初始化父类的实例属性（定义在父类构造函数中的 this.xxx）。
    class Parent {
  constructor() {
    this.parentProp = "父类实例属性"; // 👈 父类实例属性在此初始化
  }
}

class Child extends Parent {
  constructor() {
    super(); // 👈 必须调用，否则报错
    this.childProp = "子类实例属性";
  }
}

const child = new Child();
console.log(child.parentProp); // "父类实例属性"
console.log(child.childProp);  // "子类实例属性"

  2. super() 的底层行为

    • 调用 super() 时，JavaScript 引擎会先创建父类的实例（即 this 的初始值），再将该实例交给子类构造函数继续加工，此时this指向的就是子类实例。
    • 这意味着：父类构造函数负责初始化父类实例属性，子类构造函数在此基础上添加自己的属性。

二、关键细节：super() 与 this 的关系

  1. super() 必须在子类构造函数的最顶部调用
    在访问 this 之前必须调用 super()，否则会报错：

    class Child extends Parent {
  constructor() {
    this.childProp = "子类属性"; // ❌ 错误：Must call super constructor...
    super();
  }
}

  2. super() 的返回值是子类实例
    super() 返回的是父类构造函数处理后的 this 对象，该对象已经是子类的实例：

    class Child extends Parent {
  constructor() {
    const instance = super(); // super() 返回子类实例
    console.log(instance === this); // true
  }
}

        class Parent {
          constructor() {
            console.log("Parent 构造函数中的 this:", this); // this 此时是 Child 的实例
            this.parentProp = "父类属性";
            console.log("Parent 构造函数中的 this:", this); // this 此时是 Child 的实例，已包含 parentProp
          }
        }

        class Child extends Parent {
          constructor() {
            super(); // 此时 this 已经由 Parent 构造函数初始化
            console.log("Child 构造函数中的 this:", this); // this 已包含 parentProp
            this.childProp = "子类属性";
            console.log("Child 构造函数中的 this:", this); //this 此时是 Child 的实例 , this 已包含 parentProp，childProp
            
          }
        }

        new Child();
        // 输出顺序：
        // "Parent 构造函数中的 this: Child {}"
        // "Parent 构造函数中的 this: Child {parentProp: '父类属性' }"
        // "Child 构造函数中的 this: Child { parentProp: '父类属性' }"
        // "Child 构造函数中的 this: Child { parentProp: '父类属性',childProp: '子类属性' }"
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