如何在创建Member类实例时内置age计算方法，不使用全局空间

下面提供几种可行的方案，既能把getAge函数纳入类内部避免全局空间污染，又能保持实例创建时自动计算年龄的逻辑：

方案1：使用静态方法（推荐） #

因为getAge仅处理传入的生日参数，不需要访问实例属性，适合定义为类的静态方法，属于类本身而非实例：

class Member {
  static id = 0;

  // 将getAge转为类的静态方法
  static getAge(birthDay) {
    const now = new Date();
    const bd = new Date(birthDay);
    const diff = now - bd;
    const age = new Date(diff).getFullYear() - 1970;

    return age;
  }

  constructor(firstName, lastName, birthDay) {
    Member.id++;
    this.id = Member.id;
    this.firstName = firstName;
    this.lastName = lastName;
    this.birthDay = birthDay;
    // 调用类的静态方法计算年龄
    this.age = Member.getAge(birthDay);
  }
}

const m1 = new Member('Oliver', 'Cruz', '11/13/1990');
console.log('m1:', m1.age); // 32 (as of Nov 13rd, 2022)

const m2 = new Member('Sophia', 'Brown', '11/30/1992');
console.log('m2:', m2.age); // 29 (as of Nov 13rd, 2022)

方案2：使用私有实例方法（ES2022+） #

如果后续需要让年龄计算逻辑访问实例的其他属性，可以用私有方法封装，外部无法访问，彻底隔离内部实现：

class Member {
  static id = 0;

  constructor(firstName, lastName, birthDay) {
    Member.id++;
    this.id = Member.id;
    this.firstName = firstName;
    this.lastName = lastName;
    this.birthDay = birthDay;
    // 调用私有方法计算年龄
    this.age = this.#getAge(birthDay);
  }

  // 私有方法，仅能在类内部访问
  #getAge(birthDay) {
    const now = new Date();
    const bd = new Date(birthDay);
    const diff = now - bd;
    const age = new Date(diff).getFullYear() - 1970;

    return age;
  }
}

const m1 = new Member('Oliver', 'Cruz', '11/13/1990');
console.log('m1:', m1.age); // 32 (as of Nov 13rd, 2022)

const m2 = new Member('Sophia', 'Brown', '11/30/1992');
console.log('m2:', m2.age); // 29 (as of Nov 13rd, 2022)

方案3：构造函数内部定义函数（不推荐） #

这种方式会为每个实例创建独立的函数副本，浪费内存，仅适合临时简单场景：

class Member {
  static id = 0;

  constructor(firstName, lastName, birthDay) {
    Member.id++;
    this.id = Member.id;
    this.firstName = firstName;
    this.lastName = lastName;
    this.birthDay = birthDay;

    // 构造函数内部定义计算函数
    const getAge = (birthDay) => {
      const now = new Date();
      const bd = new Date(birthDay);
      const diff = now - bd;
      const age = new Date(diff).getFullYear() - 1970;

      return age;
    };

    this.age = getAge(birthDay);
  }
}

内容的提问来源于stack exchange，提问作者Miu