JavaScript Fundamentals · Async JavaScript · Lesson 26 of 48
Callbacks and setTimeout
Concept
Callbacks and setTimeout
JavaScript can schedule code to run later without blocking everything else, and setTimeout is the simplest example.
This non-blocking behavior is what lets a web page stay responsive to clicks and scrolling even while something slow, like a network request, is still in progress.
By the end of this lesson
- Explain the core idea behind Callbacks and setTimeout.
- Predict output before running a change.
- Test one realistic and one unusual input.
- Use the result to make the next decision.
How to study this page
- 1. Read one concept.
- 2. Change the example.
- 3. Run, compare, and explain.
- 4. Complete the challenge below.
Syntax
setTimeout(callbackFunction, delayInMilliseconds);ExampleRunnable
console.log("Start");
setTimeout(() => console.log("This runs later"), 1000);
console.log("End");Repeating with setInterval
let count = 0;
const id = setInterval(() => {
count++;
console.log(count);
if (count === 3) clearInterval(id);
}, 1000);Note: clearInterval stops a repeating setInterval — without it, the callback would keep firing forever.
Self-check before continuing
Without looking at the example, describe what changes when you modify one input in Callbacks and setTimeout. Then reopen the editor and prove your explanation with a small test.
You are ready to continue when you can predict, test, and explain the result.
Your turn
Predict the console output order before running the code above.
Loading editor…
Console