Hidden Classes: The JavaScript performance secret that changed everything

V8's Hidden Classes optimize JavaScript performance by assigning internal classes to track object shapes and property locations. Consistent object creation patterns allow V8 to reuse these classes, enabling significant speed improvements—up to 56x faster in benchmarks. Performance pitfalls include using delete, inconsistent property order, and dynamic property creation. Writing "monomorphic" code with consistent object shapes leads to substantial optimizations. Applying these principles to a Node.js service reduced response time from 340ms to 28ms, while decreasing memory and CPU usage by 40% and 65% respectively.