Node.js

Node.js is a runtime that embeds the V8 engine and the libuv library so JavaScript can run on the server: files, networking, processes. The model is the same as in the browser — a single-threaded event loop — but on the server that single-threadedness turns from a background detail into the central constraint the whole architecture is built around.

Node shines for I/O-bound work: while it waits on the network or disk, the event loop serves thousands of other connections without blocking. And it is weak for CPU-bound work: one long synchronous function occupies the single thread and stalls all other requests until it finishes. From this grow four advanced topics: how the loop's phases work, how to step beyond one thread via worker_threads and child_process, how streams keep memory flat via backpressure, and how to shut a process down cleanly without dropping live requests. Each mechanism is broken down in the layers below.

Topic map

• The Node runtime — V8 plus libuv, the single-threaded event loop, and why Node is good for I/O and poor for CPU.
• Event loop phases — the ordered libuv phases, microtasks, and where setImmediate sits versus process.nextTick.
• Multi-core concurrency — worker_threads versus child_process, and the cluster module for spreading across cores.
• Streams and backpressure — the four stream types, chunked processing, and backpressure via pipe.
• Lifecycle and graceful shutdown — clean termination on SIGTERM with request draining and a timeout.

Common traps

Interview relevance

Node.js is the core of the backend section, and here it is the execution model — not API knowledge — that gets tested. A candidate who explains "the single thread serves I/O without blocking, but one CPU task stops it" immediately shows they grasp the essence rather than memorizing "Node is asynchronous".

Typical checks:

• Why Node is good for I/O and poor for CPU, and how not to block the event loop.
• The event loop phases and the order of setImmediate / setTimeout(0) / process.nextTick.
• The difference between worker_threads and child_process and when to reach for each.
• What streams and backpressure solve; what graceful shutdown is and why the timeout.

Common wrong answer: "Node is multi-threaded, heavy computation runs in parallel by itself". In fact application JavaScript runs on one thread; parallelism comes only from an explicit worker_threads, child_process, or cluster, and synchronous CPU work on the main thread freezes the whole server.