Javascript对象未定义问题

Question

我在这里有点困惑。有人可以告诉我为什么这有效吗:

var selectedAttrs = {"mattress_size_variation":{"displayName":"Mattress Size","value":"King","displayValue":"King"},"mattress_feel_variation":{"displayName":"Mattress Feel","value":"Soft","displayValue":"Soft"}};

var selectedAttributes = JSON.parse(selectedAttrs);
return selectedAttributes.mattress_size_variation.value.toLowerCase();

//Returns "King"

但这不会并引发错误？

var selectedAttrs = {"mattress_size_variation ":{"displayName":"Mattress Size","value":"Twin","displayValue":"Twin"}};

var selectedAttributes = JSON.parse(selectedAttrs);
return selectedAttributes.mattress_size_variation.value.toLowerCase();

TypeError: Cannot read property "value" from undefined 

有什么区别，我应该如何从最后一个获得值？我假设我需要进行某种检查，因为一个可以工作，而另一个则不能。

Answer 1

第二个 JSON 中有一个未 trim 的空格，因此您必须使用方括号访问该属性:

selectedAttributes['mattress_size_variation ']

var selectedAttrs = '{"mattress_size_variation ":{"displayName":"Mattress Size","value":"Twin","displayValue":"Twin"}}'

var selectedAttributes = JSON.parse(selectedAttrs);
console.log(selectedAttributes['mattress_size_variation '].value);