How do JavaScript virtual machines optimize JavaScript performance?

Understanding V8 (Chrome's JS engine) optimizations helps write faster JavaScript. JIT compilation: V8 interprets code initially (Ignition interpreter), then compiles frequently-run "hot" code to optimized machine code (TurboFan JIT compiler). Hidden classes (shapes): V8 creates a hidden class for each unique object shape. Creating objects with consistent property order reuses the same hidden class and enables faster property access. Inline caching: V8 caches the type of a value at a call site. Calling add(a, b) with always-number arguments is optimized. Calling with mixed types triggers deoptimization. Deoptimization triggers: adding properties to an object after creation, using delete on properties, changing property types, using arguments object. Array optimizations: typed arrays (Int32Array, Float64Array) have predictable element types — V8 stores them contiguously and uses unboxed representations. Mixing types in a regular array degrades to a slower "holey" representation. Practical implications: consistent object shapes, monomorphic functions (same types at call sites), typed arrays for numerical computation, avoiding dynamic property deletion.