Stop 23 · Interview PreparationTechnical · 6/8

Concurrency & the event loop

In short

"What does this code print?" with setTimeout and Promises is a Node.js interview favourite. Understand the event loop and you can answer it, and avoid blocking your server.

Deep understanding of how Node.js handles concurrency despite being single-threaded — a frequent, somewhat tricky interview topic that trips up candidates who've only used async/await without understanding it.

The purple blob waiter hurries between tables at a dhaba with a tray.

Think of it like this

One waiter at a busy dhaba

He takes an order, passes it to the kitchen and moves to the next table instead of standing there. When a dish is ready, he delivers it. One waiter serves twenty tables. If he starts cooking himself, everyone waits.

Watch it happen

Why A, D, C, B?

Call stack

Microtasks

Timers

Console

log("A")

1/6Plain code runs first, top to bottom. A prints.

The big ideas

What you need to know

One thread runs your JS

The event loop picks the next callback when the stack is empty. Slow I/O happens outside, in libuv and the OS.

Microtasks jump the queue

Promise callbacks run right after the current code, before timers or I/O callbacks.

Never block the loop

A long CPU loop or a sync function like fs.readFileSync in a hot path stalls every request. Use worker_threads or a queue.

Races still happen

Between two awaits, other requests run. "Check then update" logic needs atomic database operations or locks.

See it in code

Walk through the code

1console.log("A");2setTimeout(() => console.log("B"), 0);3Promise.resolve().then(() => console.log("C"));4console.log("D");5// Output: A, D, C, B6 7// Blocking: nothing else runs while this loop spins8app.get("/report", (req, res) => {9  let total = 0;10  for (let i = 0; i < 5e9; i++) total += i;   // every other request waits11  res.json({ total });12});13// Fix: move CPU-heavy work to worker_threads or a job queue

1/3Synchronous logs first, then microtasks, then timers.

While /report is computing, even a simple health check can't respond. Many unexplained timeouts in production are this.

Save yourself the pain

Common mistakes

Oops

Saying async/await makes code run in parallel threads.

Do this instead

await just pauses that function and frees the thread. The work runs concurrently, not in parallel.

Oops

Assuming two requests can't interfere because Node is single-threaded.

Do this instead

They interleave at every await. Read-modify-write on shared data still needs atomic updates.

Your mission

Predict, then run

~25 min

Quick check

What does this print? console.log(1); setTimeout(() => console.log(2), 0); Promise.resolve().then(() => console.log(3)); console.log(4);

Go deeper

1Practice explaining the event loop phases and the microtask/macrotask queue distinction.
2Be ready to predict the output order of a tricky async code snippet.
3Understand what actually blocks the event loop and what doesn't.

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