How can I change the way String or Number casts a class?

For example, I have the class User:

class User {
  constructor(name, age) {
    this._name = name;
    this._age = age;
  }
}

How do I convert a class object to String and get the name attribute back when I convert it to String. And if I want to convert the object of the class to Number, it returns the age attribute.

Like this:

const user = new User('Kevin', 23);

console.log(String(user)) // Kevin
console.log(Number(user)) // 23

Thanks!

CodePudding user response：

Use toString and valueOf methods.

class User {
  constructor(name, age) {
    this._name = name;
    this._age = age;
  }
  toString() { return this._name; }
  valueOf() { return this._age; }
}
const user = new User('Kevin', 23);

console.log(String(user)) // Kevin
console.log(Number(user)) // 23

CodePudding user response：

I want to know why? It doesn't look like this reduces complexity.

String(user);
Number(user);

user.string;
user.number;

user.name;
user.age;