V8 Engine Interview Questions for Node.js Developers

V8 Engine Interview Questions for Node.js Developers

V8 comes up in deep Node.js interviews. Here are the common questions.

What is V8 and what does it do in Node.js?

V8 is Google's JavaScript engine that compiles JavaScript to machine code using JIT compilation. Node.js uses V8 to run JavaScript on the server, which is why Node.js is fast.

What is JIT compilation in V8?

Just-in-time compilation. V8 uses an interpreter (Ignition) for fast startup and an optimizing compiler (TurboFan) for hot code. This two-tier system balances startup speed with long-term performance.

What are hidden classes in V8?

Internal structures V8 uses to track object shapes. Adding or deleting properties after creation changes the hidden class and deoptimizes code. Initializing objects with all properties at creation keeps hidden classes stable.

How does garbage collection work in V8?

V8 uses a generational garbage collector: young objects collected often (minor GC), old objects collected less often (major GC). Leaks happen from forgotten references like event listeners and unbounded caches.

What is deoptimization in V8?

When TurboFan's optimization assumptions break (a function gets a different type than expected), V8 deoptimizes back to Ignition bytecode. This ensures correctness while still optimizing common cases.

How to Answer Well

Connect V8 to performance: explain how JIT, hidden classes, and GC affect your code's speed. Show you understand why type stability and object initialization matter, not just the definitions.

The Takeaway

Know what V8 is and does, JIT compilation, hidden classes, garbage collection, and deoptimization. Connect each to performance to show you understand why these matter for writing fast Node.js code.