Anuradha Aggarwal

Posted on Mar 20, 2021 • Updated on Jan 31, 2022 • Originally published at anuradha.hashnode.dev

Debouncing v/s Throttling

#javascript #performance #100daysofcode #webdev

In this article, we will discuss what exactly is debouncing and throttling, why we use them & the main difference between the two.

There might be some functionality in a web page that requires time-consuming computations. If such a method is invoked frequently, it might greatly affect the performance of the browser, as JavaScript is a single-threaded language. So to prevent such conditions, we use the concept of debouncing and throttling. Both of these techniques are used for performance optimization and rate-limiting of certain function calls.

Now we will deep dive into these concepts using a simple example :

Let's take an example where you need to implement a suggestive text functionality into your application. Based on user input we call an expensive function (such as make an API call to the backend) and give suggestions to the user.

Case 1: General Case (Without any optimization)

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <link rel="stylesheet" href="style.css">
</head>

<body>
    <input type="text" name="search" id="search" placeholder="Search">
    <script src="index.js" type="text/javascript"></script>
</body>
</html>

//Case 1 : Without Optimization
let textField = document.querySelector('#search');
textField.addEventListener('input', () => {
    console.count("search action without optimization!!")
})

In the above code snippet, we will attach a listener to the keypress event. Every time you enter any keyword it calls a function.

The above technique is not optimal and leads to unnecessary function calls that stall the performance of the web page.

first, we'll examine do we really need to make a function call if the user is still typing? No. We should wait till the user has halted typing for at least some amount of time before we make a function call.

To optimize it further we will use debouncing & throttling.

Now let's explore them one by one:

Case 2: Debouncing Technique

In the debouncing technique, no matter how many times the user fires the event, the attached function will be executed only after the specified time once the user stops firing the event.

Let's modify your js code further:

//Case 2: With Debouncing
const debounce = (func, delay) => {
    let timerId;
    return function () {
        clearTimeout(timerId)
        timerId = setTimeout(() => func.apply(this, arguments), delay)
    };
};

function handleButtonClick() {
     callbackFn();
}

function handleConsole() {
    console.count("debounce function executed!!")
}

let callbackFn = debounce(handleConsole, 1000);

let textField = document.querySelector('#search');
textField.addEventListener('input', handleButtonClick);

The debounce() function forces a function to wait a certain amount of time before running again. The function is built to limit the number of times a function is called.

As you noticed, in the above scenario the number of function calls is decreased drastically which improves our web performance.

As compared to the first scenario, in this case, we wait for the user to stop typing for a few seconds before calling our function. thus, on every keystroke, we wait for some seconds before giving out suggestions.

Case 3: Throttling Technique

Throttling is a technique in which, no matter how many times the user fires the event, the attached function will be executed only once in a given time interval.

//Case 3: With Throttling
const throttle = (func, delay) => {
    let toThrottle = false;
    return function () {
        if (!toThrottle) {
            toThrottle = true;
            func.apply(this, arguments)
            setTimeout(() => {
                toThrottle = false
            }, delay);
        }
    };
};

function handleButtonClick() {
     callbackFn();
}

function handleConsole() {
    console.count("throttle function executed!!")
}

let callbackFn = throttle(handleConsole, 1000);

let textField = document.querySelector('#search');
textField.addEventListener('input', handleButtonClick);

Throttling is used to call a function after every millisecond or a particular interval of time only the first click is executed immediately.

throttle function takes an existing expensive function & delay limit and returns a better expensive function which is called only after a certain delay limit.

In the above scenario, if a user continues to type every function is executed after 1000ms except the first one which is executed as soon as the user starts typing. It prevents the frequent calling of the function.

But what is the difference between the two??

Difference between Debounce and Throttle

Debounce postpones execution until there is no input change for the delay period of time. If a change occurs, cancel the previously scheduled execution and create a new schedule.
Throttle allows execution immediately if the toThrottle flag is false. After the execution, this function will not be called until the delay period has lapsed.

In debouncing, it only makes an API call if the time difference between two keystrokes events is greater than a certain limit.
Whereas, in Throttling, it only makes an API call if the time difference between two function calls is greater than a certain limit.

Application:

In content-loading webpages like Facebook and Twitter where the user keeps on scrolling. In these scenarios, if the scroll event is fired too frequently, there might be a performance impact, as it contains lots of videos and images.
Wait until the user stops resizing the window
Don’t fire an ajax event until the user stops typing
Measure the scroll position of the page and respond at most every 50ms
Ensure good performance as you drag elements around in an app

Which one is better?

It totally depends on the use case and scenario where you are applying these concepts. Both of these are meant for performance optimization.