未捕获的TypeError:无法分配为只读属性 [英] Uncaught TypeError: Cannot assign to read only property

问题描述

我正在尝试Nicholas Zakas撰写的《 Web开发人员的专业JavaScript》一书中的一个非常简单的示例，但是我不知道我在这里做错了什么.一定是我错过的非常简单的事情，但是我被卡住了.

I was trying out this really simple example from the awesome "Professional JavaScript for Web Developers" book by Nicholas Zakas but I can't figure what I am doing wrong here. Must be something really simple that I missed but I'm stuck.

这是代码:

'use strict';

var book = {};

Object.defineProperties(book, {
    originYear: {
        value: 2004,
        writable: false
    },

    _year: {
        value: 2004
    },

    edition: {
        value: 1
    },

    year : {
        get: function() {
            return this._year;
        },

        set: function(newValue) {
            if(newValue > this.originYear) {
                this._year = newValue;
                this.edition += newValue - this.originYear;
            }
        }
    }
});

console.log(book.edition);
book.year = 2006;
console.log(book.edition);

我在Chrome控制台上遇到的错误是:

The error I am getting on the Chrome console is:

未捕获的TypeError:无法分配为只读#main.js:31 Object.defineProperties.year.setmain.js:39的属性"_year"(匿名函数)

有人可以解释我哪里出了问题吗?

Can someone please explain where I have gone wrong?

这是小提琴

推荐答案

使用 Object.defineProperties 时，默认情况下 writable 设置为 false ，因此 _year 和版本实际上是只读属性.

When you use Object.defineProperties, by default writable is set to false, so _year and edition are actually read only properties.

将它们明确设置为可写:true :

_year: {
    value: 2004,
    writable: true
},

edition: {
    value: 1,
    writable: true
},

签出 此方法的MDN .

可写
true ，并且仅当与该属性关联的值可以使用赋值运算符进行更改时.
默认为 false .

writable
true if and only if the value associated with the property may be changed with an assignment operator.
Defaults to false.

这篇关于未捕获的TypeError:无法分配为只读属性的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！