A value property returns a generally constant value. Examples are the NaN and Infinity values returned by core objects.
Do not create your own object properties with the same name as internal or core value properties. If you do, unpredictable behavior may result. Internal values should be protected against accidental damage in this way and they would be, if the interpreter designers were infallible. Unfortunately, to date, they have proven to everyone's dissatisfaction that infallibility is a lost art.