Javascript JS/ES6：未定义的解构
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JS/ES6: Destructuring of undefined
提问by user3142695
I'm using some destructuring like this:
我正在使用一些这样的解构：
const { item } = content
console.log(item)
But how should I handle content === undefined- which will throw an error?
但是我应该如何处理content === undefined- 这会引发错误？
The 'old' way would look like this:
“旧”方式如下所示：
const item = content && content.item
So, if contentis undefined -> itemwill also be undefined.
因此，如果content未定义 ->item也将是未定义的。
Can I do something similar using destructuring?
我可以使用解构做类似的事情吗？
回答by Ori Drori
You can use short circuit evaluationto supply a default if contentis a falsy value, usually undefinedor nullin this case.
您可以使用短路评估来提供默认值 if contentis a falsy value，通常undefined或null在这种情况下。
const content = undefined
const { item } = content || {}
console.log(item) // undefinedA less idiomatic (see this comment) way is to spread the content into an object before destructuring it, because nulland undefinedsvalues are ignored.
一种不太惯用的方法（请参阅此评论）是在解构之前将内容传播到对象中，因为null和undefineds值被忽略。
const content = undefined
const { item } = { ...content }
console.log(item) // undefinedIf you are destructuring function params you can supply a default (= {}in the example).
如果您正在解构函数参数，您可以提供一个默认值（= {}在示例中）。
Note:The default value would only be applied if the destructured param is undefined, which means that destructuring nullvalues will throw an error.
注意：只有在解构参数为 时才会应用默认值undefined，这意味着解构null值将引发错误。
const getItem = ({ item } = {}) => item
console.log(getItem({ item: "thing" })) // "thing"
console.log(getItem()) // undefined
try {
getItem(null)
} catch(e) {
console.log(e.message) // Error - Cannot destructure property `item` of 'undefined' or 'null'.
}Or even set a default value for the itemproperty if the input object doesn't contain the property
item如果输入对象不包含该属性，甚至可以为该属性设置一个默认值
const getItem = ({ item = "default" } = {}) => item
console.log(getItem({ item: "thing" })) // "thing"
console.log(getItem({ foo: "bar" })) // "default"回答by Nir O.
accepted answer does not work for truely undefined values which were not set by const content = undefined. in such cases this will work:
接受的答案不适用于未设置的真正未定义的值const content = undefined。在这种情况下，这将起作用：
const { item } = (typeof content !== 'undefined' && content) || {}
console.log(item)
回答by Вадик Орлов
const { item } = Object(content)
