You can see the description of how hidden classes work on the v8 wiki [1]. The short explanation is that the way v8 finds a hidden class is by following a chain

You can see the description of how hidden classes work on the v8 wiki [1]. The short explanation is that the way v8 finds a hidden class is by following a chain of transitions starting at the empty class. Intermediate classes have to be kept around to retain the transitions. That is, in code like: function Point(x, y) { this.x = x; this.y = y; } v8 retains the hidden class for {x}, so it can find the hidden class for {x, y}. [1] https://github.com/v8/v8/wiki/Design%20Elements#fast-property-access https://github.com/v8/v8/wiki/Design%20Elements#fast-propert...