JavaScript:使用 Object.defineProperties()

Question

我正在尝试学习如何使用 Object.defineProperties()。我正在使用以下代码:

var Person = function(firstName, lastName)
{
    this.firstName = firstName;
    this.lastName = lastName;
};

Object.defineProperties(Person, {
    sayHi : {
        get : function() {return "Hello";},
        enumerable : true
    },
    sayBye : {
        get : function() {return "Bye";},
        enumerable : true
    }
});


var john = new Person('John', 'Doe');
console.log(john.sayHi());

但我不断得到:

TypeError: john.sayHi is not a function
console.log(john.sayHi());

有人可以告诉我这段代码有什么问题吗？

谢谢

Answer 1

嗯，您没有将 sayHi 定义为函数。这是将其定义为函数的方法:

var Person = function(firstName, lastName)
{
    this.firstName = firstName;
    this.lastName = lastName;
};
// Define the properties on the prototype, not the Person object itself
Object.defineProperties(Person.prototype, {
    sayHi : {
        get : function() {
          return function() {
            return "Hello, I am " + this.firstName + " " + this.lastName;
          };
        },
        enumerable : true
    },
    sayBye : {
        get : function() {
          return function() {
            return "Bye";
          };
        },
        enumerable : true
    }
});

var john = new Person('John', 'Doe');
console.log(john.sayHi());
console.log(john.sayBye());

准确地说:在您的代码中，john.sayHi 返回“Hello”字符串，它是一个字符串基元，因此绝对不是一个函数;-)

属性的获取函数必须返回一个函数才能实现您想要的功能。

为了给您更长的答案，请参阅以下其他实现，充分利用两件事:首先是 ES5 (Object.create()) 和 ES6 (Object .defineProperties())和JS的原型(prototype)性质(不使用new运算符，原型(prototype)继承):

var Person = {
    init: function(firstName, lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }
};

Object.defineProperties(Person, {
    sayHi : {
        get : function() {return function() {return "Hello, I am " + this.firstName + " " + this.lastName;}},
        enumerable : true
    },
    sayBye : {
        get : function() {return function() {return "Bye";};},
        enumerable : true
    }
});

var Employee = Object.create(Person); // Employee inherits from Person

Employee.init = function(firstName, lastName, position) {
    this.firstName = firstName;
    this.lastName = lastName;
    this.position = position;
};

Object.defineProperties(Employee, {
    introduce : {
        get : function() {return function() {
            return this.sayHi() + ", " + this.position;
        }},
        enumerable : true
    },
    farewell : {
        get: function() {return function() {
            return this.sayBye() + ", it was a pleasure to meet you";
        }},
        enumerable: true
    }
});

var john = Object.create(Employee); // john inherits from Employee
john.init('John', 'Doe', 'Manager');

console.log(john.sayHi()); // Inherited from Person
console.log(john.introduce()); // Inherited from Employee
console.log(john.sayBye()); // Inherited from Person
console.log(john.farewell()); // Inherited from Employee

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