【Javascript Property Names】
Property names must be strings. This means that non-string objects cannot be used as keys in the object. Any non-string object, including a number, is typecasted into a string via the toString method.
This outputs "value", since 1 is type-casted into '1'.
This also outputs "value", since both foo and bar are converted to the same string. In the SpiderMonkeyJavaScript engine, this string would be "['object Object']".
参考：https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Property_Accessors
