Debouncing vs Throttling: When to Use Each

While I was looking over some event handler code the other day, I came across a search bar that was making API calls on every single keystroke. I watched the network tab in disbelief as it fired off dozens of requests while the user typed "JavaScript". Little did I know this was going to lead me down a rabbit hole of performance optimization that I should have explored years ago.

Understanding the Event Handler Performance Problem

I was once guilty of writing code like this:

searchInput.addEventListener('input', (e) => {
  fetchSearchResults(e.target.value);
});

Innocent enough, right? Wrong. Every keystroke triggered a network request. Type "debouncing" and that's 10 API calls. The server was getting hammered, the UI was laggy, and users were frustrated with outdated results popping up after they'd already finished typing.

When I finally decided to investigate, I realized that most developers face this same problem across different scenarios: scroll events, window resizing, autocomplete inputs, and more. The browser can fire these events hundreds of times per second, and if your handler does anything expensive, you're in trouble.

This is exactly where debouncing and throttling come in. These aren't just fancy terms to drop in interviews—they're practical solutions to real performance problems.

What is Debouncing? The 'Wait Until Quiet' Pattern

Debouncing is like waiting for someone to finish talking before you respond. The function doesn't execute until the user has stopped triggering the event for a specified amount of time.

Think of it this way: imagine you're in an elevator. Every time someone presses the button, the timer resets. The elevator only moves after no one has pressed the button for 3 seconds. That's debouncing.

In other words, debouncing groups multiple sequential calls into a single call that happens after a "quiet period". If events keep firing, the timer keeps resetting, and the function never executes until things calm down.

For my search bar problem, debouncing meant the API call would only fire after the user stopped typing for, say, 300 milliseconds. Type "JavaScript" quickly, and you get one API call instead of ten. Wonderful!

What is Throttling? The 'Rate Limiter' Pattern

Throttling is different. Instead of waiting for a quiet period, it guarantees that a function executes at most once per specified time interval, no matter how many times the event fires.

Using the elevator analogy again: with throttling, the elevator moves every 3 seconds regardless of how many times people press the button during that interval. Press it 50 times in 3 seconds? Still just one trip.

Luckily we can use throttling for scenarios where you need regular updates but want to prevent excessive calls. Scroll events are the perfect example. You want to update a "scroll progress" indicator smoothly, but you don't need to calculate it on every single pixel scrolled.

Implementing Debounce and Throttle from Scratch

Here's a debounce implementation I use frequently:

function debounce<T extends (...args: any[]) => any>(
  func: T,
  delay: number
): (...args: Parameters<T>) => void {
  let timeoutId: ReturnType<typeof setTimeout> | null = null;
 
  return function (this: any, ...args: Parameters<T>) {
    // Clear the previous timer
    if (timeoutId) {
      clearTimeout(timeoutId);
    }
 
    // Set a new timer
    timeoutId = setTimeout(() => {
      func.apply(this, args);
      timeoutId = null;
    }, delay);
  };
}
 
// Usage example for search
const searchInput = document.querySelector('#search') as HTMLInputElement;
 
const handleSearch = debounce((value: string) => {
  console.log('Searching for:', value);
  fetchSearchResults(value);
}, 300);
 
searchInput.addEventListener('input', (e) => {
  handleSearch((e.target as HTMLInputElement).value);
});

And here's my throttle implementation:

function throttle<T extends (...args: any[]) => any>(
  func: T,
  limit: number
): (...args: Parameters<T>) => void {
  let inThrottle: boolean = false;
  let lastResult: ReturnType<T>;
 
  return function (this: any, ...args: Parameters<T>) {
    if (!inThrottle) {
      lastResult = func.apply(this, args);
      inThrottle = true;
 
      setTimeout(() => {
        inThrottle = false;
      }, limit);
    }
 
    return lastResult;
  };
}
 
// Usage example for scroll tracking
const handleScroll = throttle(() => {
  const scrollPercent = (window.scrollY / document.body.scrollHeight) * 100;
  console.log('Scroll progress:', scrollPercent.toFixed(2) + '%');
  updateScrollIndicator(scrollPercent);
}, 100);
 
window.addEventListener('scroll', handleScroll);

The key difference is clear when you look at the implementation. Debounce clears and resets its timer with every call. Throttle sets a flag and ignores subsequent calls until the time limit passes.

Side-by-Side Comparison: When to Choose Which

I cannot stress this enough! Choosing the wrong pattern can make your UX worse, not better.

Use Debouncing When:

• You only care about the final value after user input stops
• Network requests based on typing (search, autocomplete)
• Form validation that doesn't need to run on every keystroke
• Window resize handlers where you only need the final dimensions
• Text editors with auto-save features

Use Throttling When:

• You need regular updates during continuous events
• Scroll position tracking for progress indicators
• Mouse movement tracking for animations
• Button clicks that trigger animations (prevent spam)
• Real-time data updates at controlled intervals

Here's the mental model I use: if you're waiting for the user to "finish" an action, use debounce. If you're sampling an ongoing action at regular intervals, use throttle.

Real-World Use Cases: Search Bars, Scroll Events, and Resize Handlers

Let's look at three scenarios I encounter constantly.

Search Bar with Autocomplete: This is debouncing territory. When I implemented this for a recent project, switching from instant API calls to a 300ms debounce reduced our server load by 85%. Users got better results because the search only happened after they finished typing their query.

Infinite Scroll Detection: I use throttling here. You want to check if the user is near the bottom of the page regularly, but checking on every scroll event pixel is overkill. A throttle of 200ms gives you smooth loading without hammering the scroll handler.

Window Resize for Responsive Layouts: Debouncing wins here. When I was implementing a dashboard with responsive charts, I initially used throttle. The charts kept re-rendering during the resize, causing visual jitter. Switching to debounce meant the charts only re-rendered once the user finished resizing—much smoother.

Common Pitfalls and How to Avoid Them

I've made these mistakes so you don't have to:

Pitfall #1: Losing the this Context When I first implemented debounce, I forgot to preserve the this binding. This breaks when you're debouncing class methods. That's why my implementations use func.apply(this, args).

Pitfall #2: Not Handling Leading Edge Calls Sometimes you want the function to fire immediately on the first call, then debounce subsequent calls. My basic implementation doesn't handle this. In production, I often add a leading option.

Pitfall #3: Memory Leaks with Event Listeners If you're adding debounced or throttled listeners, remember to clean them up. Store the wrapped function in a variable so you can remove it later:

const debouncedHandler = debounce(handleInput, 300);
element.addEventListener('input', debouncedHandler);
 
// Later, when cleaning up:
element.removeEventListener('input', debouncedHandler);

Pitfall #4: Choosing the Wrong Delay Value 300ms works great for search, but it's too slow for scroll events. I've learned through testing that 100-200ms feels responsive for throttling, while 300-500ms works for debouncing user input.

Choosing the Right Pattern for Your Application

When you're deciding between these patterns, ask yourself one question: "Do I need updates during the event, or only after it's done?"

During the event? Throttle. After it's done? Debounce.

For many of my projects, I actually use both. Search inputs get debounced, scroll tracking gets throttled, and resize handlers get debounced. They solve different problems and using them together gives you fine-grained control over event handling performance.

The performance gains are real. In a recent e-commerce project, implementing these patterns reduced our JavaScript execution time by 60% during peak user interactions. Page interactions felt snappier, and we saw measurable improvements in our Core Web Vitals scores.

And that concludes the end of this post! I hope you found this valuable and look out for more in the future!