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JavaScript Throttle Function Explained

In the fast-paced world of web development, ensuring smooth and responsive user experiences is paramount. But sometimes, our code can get a little overzealous, leading to unnecessary function calls and performance bottlenecks. This is where JavaScript throttle comes.

JavaScript Throttle is a powerful technique that allows you to control the execution frequency of a function. This article will guide you about JavaScript throttling. We'll explore how it works, its implementation techniques, and practical examples to put throttling to work in your projects.

What is JavaScript Throttle Function?

JavaScript throttling is a technique used to control how often a function can be called within a specific timeframe. It ensures that the function is executed at most once during that time, even if it's called multiple times.

JavaScript Throttle helps improve web application performance and responsiveness by preventing functions from being called too frequently. This can be especially useful for functions that perform expensive operations, like fetching data from an API or running complex calculations.

Consider the below image where:

  • function execution: This section represents the execution of the function being throttled.

  • Clicks: These represent events that trigger the function call.

  • Successful Clicks:  These are the clicks that successfully trigger the function execution within the allowed interval.

  • Blocked Clicks:  Clicks that occurred during the blocking delay. They are queued to be called later.

  • Blocking Delay: Defined period during which the function execution is blocked after it's been called. Any clicks that occur within this delay are blocked from immediate execution.

Imagine you have a function that performs a heavy computation, like an image resize.  The function is called immediately without throttling, every time the user clicks a button, potentially causing performance issues.

Set a limit by implementing throttling on how often the function can be called. For instance, you might decide that the function can only be executed once every 500 milliseconds.

In the image:

  1. The first click triggers the function execution (Successful Click).

  2. Subsequent clicks within the blocking delay (Blocked Clicks) are queued.

  3. Once the blocking delay is over, the next queued click triggers the function execution again (Successful Click).

  4. This process repeats, ensuring the function is called only once within the set interval regardless of how many clicks occur.

How JavaScript Throttling Works?

Throttling in JavaScript is achieved by wrapping a function with a JavaScript throttle function. This wrapper function acts as a gatekeeper, ensuring the original function is executed at most once within a specific time interval, even if called multiple times.

Wrapping a Function with Throttling

Here's how the wrapping works:

function throttle(originalFunction, delay) {
  let timeout = null;

  return function() {
    // Arguments (if any) passed to the throttled function are passed here
    const args = arguments;

    if (timeout === null) {
      // Call the original function immediately
      originalFunction.apply(this, args); // Apply allows for context passing

      timeout = setTimeout(() => {
        timeout = null;
      }, delay);

throttle Function:

  • This function takes two arguments:

  • originalFunction: The function to be throttled.

  • delay: The time interval (in milliseconds) between allowed executions.

  • It creates a variable timeout to store the reference to a pending timeout, initialized to null.

Returned Wrapper Function:

  • The throttle function returns a new anonymous function (the wrapper function). This wrapper function handles the throttling logic.

Wrapper Function Logic:

  • It captures the arguments passed to the throttled function using const args = arguments;. This allows us to pass these arguments to the original function later.

  • It checks if there's a pending timeout using if (timeout === null).

  • Immediate Execution (no pending timeout):

  • If there's no pending timeout (timeout === null):

  • The original function is called using originalFunction.apply(this, args).

  • apply is used here to pass the captured arguments (args) to the original function. You can also use originalFunction(...args) for modern JavaScript (ES6+).

  • this inside the wrapper function is different from the original function's context. apply allows you to specify the context (this) for the original function call (optional).

  • A new timeout is set using setTimeout(...) to trigger the reset logic after the delay milliseconds. This timeout will clear the timeout variable, allowing for a potential execution on the next call.

  • Ignored Call (pending timeout):

  • The current call is ignored if a timeout is pending (timeout !== null). This prevents redundant executions within the specified time interval.

Code Example

Imagine you have a search bar in your web application. As the user types their search query, you want to update the search results dynamically. However, if you call an API endpoint for every keystroke, it can lead to:

  • Excessive API calls can slow down the user experience and overload your server.

  • Unnecessary DOM updates: The search results might flicker or update too frequently, making it harder for users to type.

JavaScript Throttle can solve this problem by ensuring the search API is called at most once within a specific interval (e.g., 250 milliseconds) after the user stops typing.

function throttle(func, delay) {
  let timeout = null;

  return function() {
    const args = arguments; // Capture arguments

    if (timeout === null) {
      func.apply(this, args); // Execute function immediately
      timeout = setTimeout(() => {
        timeout = null;
      }, delay);

// Search function simulating an API call (replace with your actual logic)
function search(query) {
  console.log("Search API called for:", query);
  // Simulate API call (e.g., fetch data from server)
  for (let i = 0; i < 1000; i++) {} // Simulate some work

// Throttled search function with a delay of 250 milliseconds
const throttledSearch = throttle(search, 250);

const searchInput = document.getElementById("search-input");

searchInput.addEventListener("keyup", function() {
  const query = this.value;


  1. throttle Function: This function remains the same as before, handling the core throttling logic.

  2. search Function: This simulates a search API call (replace with your actual logic). It takes the search query as an argument and logs a message to the console.

  3. throttledSearch: The throttle function is used to create a throttled version of the search function with a delay of 250 milliseconds.

  4. Search Input Event Listener: We attach a keyup event listener to the search input element.

  5. Throttled Search Call: Inside the event listener, we capture the current search query from the input element's value and call the throttledSearch function with the query as an argument.

Now, whenever the user types in the search bar, the throttledSearch function ensures that the search API is called at most once every 250 milliseconds after the user stops typing, improving performance and user experience.

Benefits of using JavaScript Throttle

Improved Performance:

  • By limiting function calls, throttling reduces the overall workload on your web application. This is crucial for expensive functions that involve resource-intensive tasks like:

  • Fetching data from APIs

  • Performing complex calculations

  • Updating the DOM frequently (especially during animations or user interactions)

  • Reduced workload translates to smoother application performance and faster response times.

Reduced Resource Consumption:

  • Throttling prevents redundant function calls, minimizing unnecessary resource usage. This is particularly beneficial for:

  • Battery life conservation on mobile devices

  • Server load reduction in server-side applications

  • Memory optimization by avoiding redundant function executions

Enhanced User Experience:

  • Throttling contributes to a more responsive and fluid user experience by:

  • Preventing UI lags caused by excessive function calls

  • Ensuring smooth interactions, especially during rapid user actions

When to Use Throttling

Throttling is a suitable choice for scenarios where:

Frequent Function Calls Occur:

  • If a function is likely to be triggered multiple times frequently, throttling guarantees that it executes only at predefined intervals, preventing overwhelming performance impact.

Real-Time Updates Are Required:

  • JavaScript Throttle is ideal for situations where you need to provide continuous updates while maintaining performance. Examples include:

  • User input events like scrolling, resizing, or mouse movement

  • Real-time data visualizations

  • Live game mechanics or simulations

Multiple Calls Produce Similar Results:

  • If rapid function calls within a short timeframe result in the same outcome, throttling prevents redundant executions and optimizes resource utilization.


JavaScript throttle acts as a smooth and efficient working of your web applications.

By throttling, you gain:

  • Boosted performance: No more lagging code!

  • Happy users: Responsive interactions keep users engaged.

  • Efficient resource use: JavaScript Throttle prevents overloading the browser.


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