javascript - 遍历执行switch语句的数组返回TypeError：无法分配给对象'#的只读属性'location'<object>' <div id="body"><p>I have a read only array that i copied to become a mutable array <code>let mutableForecast = [...forecast]</code> I am taking that new array and iterating through it with <code

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标签： javascriptarraysswitch-statement

如果不知道从哪里来就很难确定forecast，但我怀疑问题在于数组的元素不是普通对象，而是定义为不可变的自定义类型的实例。您的第三个链接有可能的解决方案。关键是您不能简单地通过以这种方式使用 rest & spread 将不可变数组转换为可变数组。您需要单独修改数组中每个项目的可变性。

你可能需要这样的东西：

let mutableForecast = [...forecast]
mutableForecast.forEach((obj, i) => {
    // make this element's location property mutable
    Object.defineProperty(obj, 'location', { writable: true })
    
    // calculate and set new value
    switch (obj.location) {
        case 'BRITISH_COLUMBIA':
            obj.location = 'BC'
            break;
        default:
            obj.location = 'other'
    }
})

这也可能有效，我认为它更清洁。您必须尝试一下才能确定：

let mutableForecast = Array.from(forecast)
.map(forecastItem => ({
    ...forecastItem,
    location: getShortLocation(forecastItem.location)
}))

function getShortLocation( sourceLocation ) {
    switch (sourceLocation) {
        case 'BRITISH_COLUMBIA': return 'BC'
        default:                 return 'other'
    }
}

我们正在解决的核心问题是，无论包给你什么forecast，它显然都在传输一些自定义数据类型，其中一些属性被定义为不可变。当您记录对象时，该事实不会出现，当您将类似数组的容器转换为数组时，它也不会改变。

这是因为[...forecast]不编辑项目，它只是将它们从一个数据结构原样复制到另一个数据结构中。实际上，准确地说，它将对这些对象的引用复制到一个新数组中。如果原始对象是具有锁定属性的奇怪事物，那么您的新数组将包含具有锁定属性的奇怪事物。如果我们想更改每个元素上该属性的值，我们需要在这样做之前重新定义该属性。

考虑这样一个案例：

let myDog = {
    species: 'dog',
    name: 'Fido'
}
//> myDog { species: 'dog', name: 'Fido' }

我们可以创建另一个具有相同属性的对象，如下所示：

let congruentAnimal = {
    ...myDog
}
//> congruentAnimal { species: 'dog', name: 'Fido' }

如果相同的属性名称出现两次，引擎将只使用最后一个：

let myDog = {
    species: 'cat',
    name: 'Fido',
    species: 'dog' // this will cause cat to be ignored
}
//> myDog { name: 'Fido', species: 'dog' }

因此，我们可以在复制时通过最后重新声明这些属性来覆盖单个对象属性：

let anotherCongruentAnimal = {
    ...myDog,
    species: 'NEW DOG'
}
//> anotherCongruentAnimal { name: 'Fido', species: 'NEW DOG' }

这就是第二个片段中发生的事情。这是一个扩展版本：

// create a real array whose elements are *references* to
// the objects in the array-like forecast
let arrayOfImmutableForecasts = Array.from(forecast)

// create another real array of new objects
// whose property names, values, and metadata are
// the same as the source objects
let arrayOfMutableForecasts = arrayOfImmutableForecasts.map(originalObject => { 
    let newObject = {
        // I think this will also preserve special rules like immutability
        ...originalObject, 
        
        // before we finalize the object, we declare a new simple property
        // the engine will _drop_ the implied prop declaration from above
        // and define the prop based on this simple declaration instead
        location: 'new value'
    }
    
    return newObject
})

javascript - 遍历执行switch语句的数组返回TypeError：无法分配给对象'#的只读属性'location'

问题描述

解决方案

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