javascript - 为什么惰性 getter 使用原型(prototype)而不是类？

Question

考虑以下代码:

const defclass = prototype => {
    const constructor = prototype.constructor;
    constructor.prototype = prototype;
    return constructor;
};

const Person = defclass({
    constructor: function Person(firstname, lastname) {
        this.firstname = firstname;
        this.lastname  = lastname;
    },
    get fullname() {
        delete this.fullname; // doesn't delete on instances
        return this.fullname = this.firstname + " " + this.lastname;
    }
});

const john = new Person("John", "Doe");
const jane = new Person("Jane", "Doe");

console.log(john.fullname); // John Doe
console.log(jane.fullname); // Jane Doe

这是有效的，因为 this 上的属性赋值隐藏了不存在的 setter。

---

现在，考虑使用 ES6 类的相同代码:

class Person {
    constructor(firstname, lastname) {
        this.firstname = firstname;
        this.lastname  = lastname;
    }

    get fullname() {
        delete this.fullname; // doesn't delete on instances
        return this.fullname = this.firstname + " " + this.lastname;
    }
}

const john = new Person("John", "Doe");
const jane = new Person("Jane", "Doe");

console.log(john.fullname); // throws an error because there is no setter
console.log(jane.fullname);

它不起作用的原因在 this answer 中有解释。 .这是因为我们在原型(prototype)链中找到了属性，但它没有 setter。那么，为什么当我们使用常规原型(prototype)时不会抛出相同的错误？

注意:您可以使用 delete 关键字删除行而不影响代码的行为。

Answer 1

我确实在第一个代码中遇到了同样的错误:

const defclass = prototype => {
    const constructor = prototype.constructor;
    constructor.prototype = prototype;
    return constructor;
};

const Person = defclass({
    constructor: function Person(firstname, lastname) {
        this.firstname = firstname;
        this.lastname  = lastname;
    },
    get fullname() {
        "use strict";
//      ^^^^^^^^^^^^
        return this.fullname = this.firstname + " " + this.lastname;
    }
});

const john = new Person("John", "Doe");
const jane = new Person("Jane", "Doe");

console.log(john.fullname); // John Doe
console.log(jane.fullname); // Jane Doe

只是class代码在strict mode默认情况下。

在 sloppy 模式下，赋值不起作用但会被忽略，右边的值会从 getter 中return。再次访问 .fullname 将再次运行 getter。