JavaScript如何实现真正的私有类字段？

class BankAccount {
  #balance; // 私有字段

  constructor(initialBalance) {
    if (initialBalance < 0) {
      throw new Error("Initial balance cannot be negative.");
    }
    this.#balance = initialBalance;
  }

  deposit(amount) {
    if (amount <= 0) {
      throw new Error("Deposit amount must be positive.");
    }
    this.#balance += amount;
    console.log(`Deposited ${amount}. New balance: ${this.#balance}`);
  }

  withdraw(amount) {
    if (amount <= 0) {
      throw new Error("Withdrawal amount must be positive.");
    }
    if (this.#balance < amount) {
      throw new Error("Insufficient funds.");
    }
    this.#balance -= amount;
    console.log(`Withdrew ${amount}. New balance: ${this.#balance}`);
    return amount;
  }

  getAccountInfo() {
    // 可以在类内部访问私有字段
    return `Current balance: ${this.#balance}`;
  }
}

const myAccount = new BankAccount(1000);
myAccount.deposit(500); // Deposited 500. New balance: 1500
myAccount.withdraw(200); // Withdrew 200. New balance: 1300

// 尝试从外部访问私有字段会导致语法错误或运行时错误
// console.log(myAccount.#balance); // SyntaxError: Private field '#balance' must be declared in an enclosing class
// console.log(myAccount['#balance']); // undefined

function createCounter() {
  let count = 0; // 私有变量

  return {
    increment: function() {
      count++;
      return count;
    },
    decrement: function() {
      count--;
      return count;
    },
    getCount: function() {
      return count;
    }
  };
}

const counter = createCounter();
console.log(counter.getCount()); // 0
counter.increment();
console.log(counter.getCount()); // 1
// console.log(counter.count); // undefined，无法直接访问

const _balances = new WeakMap();

class OldBankAccount {
  constructor(initialBalance) {
    _balances.set(this, initialBalance); // 将私有数据存储在WeakMap中
  }

  deposit(amount) {
    let currentBalance = _balances.get(this);
    currentBalance += amount;
    _balances.set(this, currentBalance);
    console.log(`Deposited ${amount}. New balance: ${currentBalance}`);
  }

  getBalance() {
    return _balances.get(this);
  }
}

const oldAccount = new OldBankAccount(500);
oldAccount.deposit(100);
console.log(oldAccount.getBalance()); // 600
// console.log(oldAccount._balances); // undefined
// _balances.get(oldAccount) // 如果_balances不在当前作用域，也无法访问

class Parent {
  #privateParentField = 'parent secret';
  getPrivateParentField() {
    return this.#privateParentField;
  }
}

class Child extends Parent {
  #privateChildField = 'child secret';
  getChildAndParentPrivateFields() {
    // return this.#privateParentField; // SyntaxError: Private field '#privateParentField' must be declared in an enclosing class
    return this.#privateChildField + ' and ' + this.getPrivateParentField();
  }
}

const childInstance = new Child();
console.log(childInstance.getChildAndParentPrivateFields()); // child secret and parent secret
// console.log(childInstance.#privateChildField); // SyntaxError

// myModule.js
const internalHelper = () => "This is an internal helper."; // 私有于模块
export const publicFunction = () => {
  return "Public function using " + internalHelper();
};

// main.js
import { publicFunction } from './myModule.js';
console.log(publicFunction()); // Public function using This is an internal helper.
// console.log(internalHelper()); // ReferenceError: internalHelper is not defined

const mySecretKey = Symbol('secretKey');

class DataHolder {
  constructor(data) {
    this[mySecretKey] = data;
  }

  getSecretData() {
    return this[mySecretKey];
  }
}

const holder = new DataHolder('sensitive info');
console.log(holder.getSecretData()); // sensitive info
console.log(Object.keys(holder)); // []
console.log(Object.getOwnPropertyNames(holder)); // []
console.log(holder[mySecretKey]); // sensitive info，但需要知道这个Symbol