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Node.js Timers and Callbacks

Node never waits around: slow tasks run in the background and call you back later. setTimeout runs once, setInterval repeats, and setImmediate runs after the current work.

ExampleNode.js
console.log("1 start");

setTimeout(() => console.log("5 timeout after 10 ms"), 10);
setImmediate(() => console.log("4 immediate"));
process.nextTick(() => console.log("3 nextTick"));
Promise.resolve().then(() => console.log("3b promise"));

console.log("2 end of script");

let n = 0;
const id = setInterval(() => {
  n++;
  console.log("tick", n);
  if (n === 2) clearInterval(id);
}, 20);
Output
1 start
2 end of script
3 nextTick
3b promise
4 immediate
5 timeout after 10 ms
tick 1
tick 2

The order shows the event loop at work: the script finishes first, then nextTick and promise callbacks, then setImmediate, then timers when their time is up. (A setTimeout of 0 ms and setImmediate can run in either order, so never rely on that.)

Quick check

In what order do things happen: script end or setTimeout callback?

Finish the quick check (and any code exercise) to complete this lesson, or mark it done yourself.

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