javascript 25 lines · 6 steps

Building a debounce function in JavaScript

A closure remembers a pending timer so a function only fires after calls stop arriving.

Explained by highlit
Intermediate closures timers higher-order-functions throttling this-binding
1function debounce(fn, delay) {
2 let timeoutId = null;
3 
4 function debounced(...args) {
5 const context = this;
6 
7 if (timeoutId !== null) {
8 clearTimeout(timeoutId);
9 }
10 
11 timeoutId = setTimeout(() => {
12 timeoutId = null;
13 fn.apply(context, args);
14 }, delay);
15 }
16 
17 debounced.cancel = function () {
18 if (timeoutId !== null) {
19 clearTimeout(timeoutId);
20 timeoutId = null;
21 }
22 };
23 
24 return debounced;
25}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1A closure over timeoutId lets each debounced function privately track its own pending timer.
  2. 2Clearing the previous timer on every call ensures fn only runs once activity pauses for delay ms.
  3. 3Capturing this and args at call time, then replaying them via apply, preserves the original calling context.

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