Here's one that surprises people. Two functions with identical code can differ in performance by an order of magnitude in V8. What are hidden classes and inline caches doing to make that happen?

So V8 creates hidden classes that describe the structure of an object, which properties it has and where they sit. Objects built with the same properties in the same order share a hidden class, and that lets V8 read properties at fixed offsets instead of doing dictionary lookups. Inline caches remember which hidden class a piece of code has seen at a given property access, so the next access with the same shape stays fast. When a call site sees a lot of different shapes it becomes megamorphic and falls back to slower generic lookups. That's how two identical functions end up so different, I think. One gets objects with a consistent shape and keeps its fast path, the other gets mixed shapes and loses its optimizations. The standard advice is to build objects with a consistent structure, same properties, same order, especially in hot code paths.