setTimeout and setInterval
- Node.js course
- Intermediate
- Lesson 98/116
- 5 min
Learn timers with setTimeout and setInterval.
setTimeout
To run a function after a delay, use setTimeout. It takes two parameters: the function to call and the delay in milliseconds.
Example: setTimeout(() => console.log("Hello"), 1000); prints "Hello" after one second. setTimeout returns a timer id; you can cancel the call with clearTimeout(id). Even with delay 0 the callback runs after sync code; this shows how the event loop works. To cancel a timer you write const id = setTimeout(...); clearTimeout(id);. Timers run in the background; they do not block the main code.
setInterval and clearInterval
To run a function repeatedly at intervals, use setInterval. setInterval(fn, 1000) calls the function every second. To stop it you need clearInterval(id).
Example: let count = 0; const id = setInterval(() => { count++; console.log(count); if (count >= 3) clearInterval(id); }, 100);. This increments a counter every 100 milliseconds; it stops at 3. setInterval returns an id; you pass it to clearInterval to break the loop. Otherwise the timer runs forever and the process does not exit. Designing a stop condition for every setInterval call is a good habit.
Practice Write sequential output with setTimeout
Set a timer with setTimeout(() => console.log("Done"), 0);. Print sync output with console.log("Started"). See the output order.
Starter code
// Print "Done" with setTimeout
// write here
// Print "Started"
// write hereSample solution
setTimeout(() => console.log("Done"), 0);
console.log("Started");Expected output
Başlatıldı Bitti
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the setTimeout call
setTimeout(() => console.log("Hazır"), ____);setTimeout(() => console.log("Hazır"), 1000);Explanation: 1000 milliseconds is 1 second.
2 What does setInterval return?
const id = setInterval(fn, 1000);- Bir Promise
- A timer ID Correct answer
- Bir Date nesnesi
- Nothing
Explanation: setInterval returns a timer id.
3 Match each function to its tool
| Term | Meaning |
|---|---|
setTimeout |
One-shot delay |
setInterval |
Repeating interval |
clearTimeout |
Cancel one-shot |
clearInterval |
Cancel repeating |