Any string is allowed as an object key. From the spec:
Properties are identified using key values. A property key value is either an ECMAScript String value or a Symbol value. All String and Symbol values, including the empty String, are valid as property keys. A property name is a property key that is a String value.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…