Node.js · 01/19 6 min

The event loop, visually

Call stack, microtasks, macrotasks: why Promise callbacks beat setTimeout(0).

1console.log("A");

2setTimeout(() => console.log("B"), 0);

3Promise.resolve().then(() => console.log("C"));

4console.log("D");

console

call stack

main()

microtasks (promises)

macrotasks (timers)

1/8The script itself is the first task. It runs top to bottom on the call stack.

JavaScript runs one thing at a time on the call stack. Everything asynchronous is a callback that waits in a queue until the stack is empty.

The rules

Run the current script to completion (synchronous code first, always).
Drain all microtasks: Promise.then, await continuations, queueMicrotask.
Take one macrotask: a setTimeout, I/O callback or setImmediate.
Drain microtasks again. Repeat.

So the output of the classic puzzle above is A, D, C, B: synchronous logs first, then the promise callback, then the timer, even with a 0 ms delay.

Forgotten await: the function returns a pending Promise. if (promise) is always truthy, so a failed password check "passes".
array.forEach(async x => …) doesn't wait: forEach ignores the returned promises. Use for…of with await, or await Promise.all(array.map(…)).
Sequential awaits in a loop are slow when the calls are independent; Promise.all runs them concurrently.
Unhandled rejections: an async Express handler that throws without try/catch never reaches your error middleware in Express 4.

// ❌ returns before any save finishes
items.forEach(async (it) => { await it.save(); });
// ✅
await Promise.all(items.map((it) => it.save()));

Practise it now