英文:
Does using undefined as a property key guarantee the result will be undefined?
问题
在通过变量检索对象属性的情况下，例如：
myObject[someField]
存在 someField（它是一个字符串）可能为 undefined（可能是未初始化的字符串值的结果）的可能性。我的实验表明，对于我能想到的所有类型的对象，结果都是 undefined，即：
anyObject[undefined] === undefined
这是一个众所周知的行为吗？我可以依赖这个吗？在相关文档中似乎找不到相关信息，我的备选方案是重写上述代码为：
someField ? myObject[someField] : undefined;
但如果能保证在尝试访问属性 undefined 时始终返回 undefined，我真的更喜欢简洁的方式。
英文:
In a situation where an object property is retrieved through a variable i.e.:
myObject[someField]
there is the possibility of someField (which is a string) to be undefined (the result of a potentially uninitialized string value). My experiments show that for all types of objects I can think of the result is undefined i.e.:
anyObject[undefined] === undefined
Is this a well known behavior, something I can rely on? Can't seem to find something in related documentation and my alternative would be to re-write the above as
someField ? myObject[someField] : undefined;
but would really prefer the succinct way if there is a guarantee that undefined is returned whenever we try to access the property undefined.
答案1
得分: 3
No, accessing obj[undefined] will not always return undefined. Like any value used as property name, undefined will get coerced to a string (unless it is a symbol), so it will actually access a property named "undefined". obj[undefined] is equivalent to obj["undefined"] or obj.undefined. It will return the property value if such a property exists, e.g. when obj = {undefined: true};.
You should indeed write
someField != null ? myObject[someField] : undefined;
if someField: undefined | string.
英文:
No, accessing obj[undefined] will not always return undefined. Like any value used as property name, undefined will get coerced to a string (unless it is a symbol), so it will actually access a property named "undefined". obj[undefined] is equivalent to obj["undefined"] or obj.undefined. It will return the property value if such a property exists, e.g. when obj = {undefined: true};.
You should indeed write
someField != null ? myObject[someField] : undefined;
if someField: undefined | string.
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