Why do hidden classes matter for performance?

V8 assigns a hidden class to each object to track its shape (which properties it has). When you add or delete a property, V8 creates a new hidden class. Hidden classes are how V8 optimizes property access for speed.

Initialize objects with all their properties at creation. Do not add properties later. Do not use delete. This keeps the hidden class stable, which lets V8 optimize property access and function performance.

Only if created the same way: same properties in the same order. Two objects created differently (different property order) create different hidden classes, which prevents V8 from sharing optimized code between them.