Is JavaScript compiled or interpreted in modern engines?
Both. Modern JS engines use JIT (Just-In-Time) compilation: they interpret bytecode (Ignition) for fast startup and compile hot code to machine code (TurboFan) for performance. It is not purely interpreted or purely compiled.
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Internal labels assigned to objects based on their shape (property names and order). Objects with the same hidden class share optimized code. Different shapes cause deoptimization, which hurts performance. Keep object shapes consistent.
No. The event loop is part of the host environment (browser or Node.js/libuv). V8 provides the call stack and heap. The host provides the event loop, Web APIs, and queues for async behavior.
Generational mark-and-sweep (Orinoco). Young generation collected frequently (scavenge). Old generation collected less frequently (mark-sweep-compact). Incremental and concurrent to minimize pauses.
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