Javascript 对象未定义问题

Question

I'm a little stumped here. Can someone tell me why this works:

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"

But this does not and throws an error?

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 

What is the difference and how should I get the value from the last one? I'm assuming I need to do some sort of check since one works and the other does not.

Answer 1

There's a untrimmed space in your second JSON so you have to access that property using square brackets:

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);