尝试在map函数中添加字段时出现"object is not extensible“错误

Question

我正在尝试使用map将新字段添加到数组中的项：

const newArray = oldArray.map(item => {
    return (item.newField = 'Something');
});

我试过了：

const newArray = oldArray.map(item => {
    item.newField = 'Something';
    return item;
});

然而，我得到一个错误：

TypeError: Cannot add property newField, object is not extensible

Answer 1

最有可能的情况是，对象被标记为不可扩展，并且您正在运行严格模式。

看看https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cant

当调用Object.preventExtensions(obj)方法时，您会得到错误。

'use strict';

var obj = {};
Object.preventExtensions(obj);

obj.x = 'foo';

您将看到错误Uncaught TypeError: Cannot add property x, object is not extensible

Answer 2

const newArray = oldArray.map(item => {
  return Object.assign({}, item, { newField: 'Something' });
});

Answer 3

您可以按如下方式使用对象扩散(es6功能)：

const newArray = oldArray.map(item => {
    // { ...item } creates a new object and spreads all of "item" items
    // into it. We can then assign a "newField" or overwrite "newField" 
    // if it already exists on "item"
    return { ...item, newField: 'something' };
});