Can someone explain the "debounce" function in Javascript

Question

The code in the question was altered slightly from the code in the link. In the link, there is a check for (immediate && !timeout) BEFORE creating a new timout. Having it after causes immediate mode to never fire. I have updated my answer to annotate the working version from the link.

function debounce(func, wait, immediate) {
  // 'private' variable for instance
  // The returned function will be able to reference this due to closure.
  // Each call to the returned function will share this common timer.
  var timeout;

  // Calling debounce returns a new anonymous function
  return function() {
    // reference the context and args for the setTimeout function
    var context = this,
      args = arguments;

    // Should the function be called now? If immediate is true
    //   and not already in a timeout then the answer is: Yes
    var callNow = immediate && !timeout;

    // This is the basic debounce behaviour where you can call this 
    //   function several times, but it will only execute once 
    //   [before or after imposing a delay]. 
    //   Each time the returned function is called, the timer starts over.
    clearTimeout(timeout);

    // Set the new timeout
    timeout = setTimeout(function() {

      // Inside the timeout function, clear the timeout variable
      // which will let the next execution run when in 'immediate' mode
      timeout = null;

      // Check if the function already ran with the immediate flag
      if (!immediate) {
        // Call the original function with apply
        // apply lets you define the 'this' object as well as the arguments 
        //    (both captured before setTimeout)
        func.apply(context, args);
      }
    }, wait);

    // Immediate mode and no wait timer? Execute the function..
    if (callNow) func.apply(context, args);
  }
}

/////////////////////////////////
// DEMO:

function onMouseMove(e){
  console.clear();
  console.log(e.x, e.y);
}

// Define the debounced function
var debouncedMouseMove = debounce(onMouseMove, 50);

// Call the debounced function on every mouse move
window.addEventListener('mousemove', debouncedMouseMove);

The important thing to note here is that debounce produces a function that is "closed over" the timeout variable. The timeout variable stays accessible during every call of the produced function even after debounce itself has returned, and can change over different calls.

The general idea for debounce is the following:

1. Start with no timeout.
2. If the produced function is called, clear and reset the timeout.
3. If the timeout is hit, call the original function.

The first point is just var timeout;, it is indeed just undefined. Luckily, clearTimeout is fairly lax about its input: passing an undefined timer identifier causes it to just do nothing, it doesn't throw an error or something.

The second point is done by the produced function. It first stores some information about the call (the this context and the arguments) in variables so it can later use these for the debounced call. It then clears the timeout (if there was one set) and then creates a new one to replace it using setTimeout. Note that this overwrites the value of timeout and this value persists over multiple function calls! This allows the debounce to actually work: if the function is called multiple times, timeout is overwritten multiple times with a new timer. If this were not the case, multiple calls would cause multiple timers to be started which all remain active - the calls would simply be delayed, but not debounced.

The third point is done in the timeout callback. It unsets the timeout variable and does the actual function call using the stored call information.

The immediate flag is supposed to control whether the function should be called before or after the timer. If it is false, the original function is not called until after the timer is hit. If it is true, the original function is first called and will not be called any more until the timer is hit.

However, I do believe that the if (immediate && !timeout) check is wrong: timeout has just been set to the timer identifier returned by setTimeout so !timeout is always false at that point and thus the function can never be called. The current version of underscore.js seems to have a slightly different check, where it evaluates immediate && !timeout before calling setTimeout. (The algorithm is also a bit different, e.g. it doesn't use clearTimeout.) That's why you should always try to use the latest version of your libraries. :-)

Debounced functions do not execute when invoked, they wait for a pause of invocations over a configurable duration before executing; each new invocation restarts the timer.

Throttled functions execute and then wait a configurable duration before being eligible to fire again.

Debounce is great for keypress events; when the user starts typing and then pauses you submit all the key presses as a single event, thus cutting down on the handling invocations.

Throttle is great for realtime endpoints that you only want to allow the user to invoke once per a set period of time.

Check out Underscore.js for their implementations too.

I wrote a post titled Demistifying Debounce in JavaScript where I explain exactly how a debounce function works and include a demo.

I too didn't fully understand how a debounce function worked when I first encountered one. Although relatively small in size, they actually employ some pretty advanced JavaScript concepts! Having a good grip on scope, closures and the setTimeout method will help.

With that said, below is the basic debounce function explained and demoed in my post referenced above.

The finished product

// Create JD Object
// ----------------
var JD = {};

// Debounce Method
// ---------------
JD.debounce = function(func, wait, immediate) {
    var timeout;
    return function() {
        var context = this,
            args = arguments;
        var later = function() {
            timeout = null;
            if ( !immediate ) {
                func.apply(context, args);
            }
        };
        var callNow = immediate && !timeout;
        clearTimeout(timeout);
        timeout = setTimeout(later, wait || 200);
        if ( callNow ) { 
            func.apply(context, args);
        }
    };
};

The explanation

// Create JD Object
// ----------------
/*
    It's a good idea to attach helper methods like `debounce` to your own 
    custom object. That way, you don't pollute the global space by 
    attaching methods to the `window` object and potentially run in to
    conflicts.
*/
var JD = {};

// Debounce Method
// ---------------
/*
    Return a function, that, as long as it continues to be invoked, will
    not be triggered. The function will be called after it stops being 
    called for `wait` milliseconds. If `immediate` is passed, trigger the 
    function on the leading edge, instead of the trailing.
*/
JD.debounce = function(func, wait, immediate) {
    /*
        Declare a variable named `timeout` variable that we will later use 
        to store the *timeout ID returned by the `setTimeout` function.

        *When setTimeout is called, it retuns a numeric ID. This unique ID
        can be used in conjunction with JavaScript's `clearTimeout` method 
        to prevent the code passed in the first argument of the `setTimout`
        function from being called. Note, this prevention will only occur
        if `clearTimeout` is called before the specified number of 
        milliseconds passed in the second argument of setTimeout have been
        met.
    */
    var timeout;

    /*
        Return an anomymous function that has access to the `func`
        argument of our `debounce` method through the process of closure.
    */
    return function() {

        /*
            1) Assign `this` to a variable named `context` so that the 
               `func` argument passed to our `debounce` method can be 
               called in the proper context.

            2) Assign all *arugments passed in the `func` argument of our
               `debounce` method to a variable named `args`.

            *JavaScript natively makes all arguments passed to a function
            accessible inside of the function in an array-like variable 
            named `arguments`. Assinging `arguments` to `args` combines 
            all arguments passed in the `func` argument of our `debounce` 
            method in a single variable.
        */
        var context = this,   /* 1 */
            args = arguments; /* 2 */

        /*
            Assign an anonymous function to a variable named `later`.
            This function will be passed in the first argument of the
            `setTimeout` function below.
        */
        var later = function() {

            /*      
                When the `later` function is called, remove the numeric ID 
                that was assigned to it by the `setTimeout` function.

                Note, by the time the `later` function is called, the
                `setTimeout` function will have returned a numeric ID to 
                the `timeout` variable. That numeric ID is removed by 
                assiging `null` to `timeout`.
            */
            timeout = null;

            /*
                If the boolean value passed in the `immediate` argument 
                of our `debouce` method is falsy, then invoke the 
                function passed in the `func` argument of our `debouce`
                method using JavaScript's *`apply` method.

                *The `apply` method allows you to call a function in an
                explicit context. The first argument defines what `this`
                should be. The second argument is passed as an array 
                containing all the arguments that should be passed to 
                `func` when it is called. Previously, we assigned `this` 
                to the `context` variable, and we assigned all arguments 
                passed in `func` to the `args` variable.
            */
            if ( !immediate ) {
                func.apply(context, args);
            }
        };

        /*
            If the value passed in the `immediate` argument of our 
            `debounce` method is truthy and the value assigned to `timeout`
            is falsy, then assign `true` to the `callNow` variable.
            Otherwise, assign `false` to the `callNow` variable.
        */
        var callNow = immediate && !timeout;

        /*
            As long as the event that our `debounce` method is bound to is 
            still firing within the `wait` period, remove the numerical ID  
            (returned to the `timeout` vaiable by `setTimeout`) from 
            JavaScript's execution queue. This prevents the function passed 
            in the `setTimeout` function from being invoked.

            Remember, the `debounce` method is intended for use on events
            that rapidly fire, ie: a window resize or scroll. The *first* 
            time the event fires, the `timeout` variable has been declared, 
            but no value has been assigned to it - it is `undefined`. 
            Therefore, nothing is removed from JavaScript's execution queue 
            because nothing has been placed in the queue - there is nothing 
            to clear.

            Below, the `timeout` variable is assigned the numerical ID 
            returned by the `setTimeout` function. So long as *subsequent* 
            events are fired before the `wait` is met, `timeout` will be 
            cleared, resulting in the function passed in the `setTimeout` 
            function being removed from the execution queue. As soon as the 
            `wait` is met, the function passed in the `setTimeout` function 
            will execute.
        */
        clearTimeout(timeout);

        /*
            Assign a `setTimout` function to the `timeout` variable we 
            previously declared. Pass the function assigned to the `later` 
            variable to the `setTimeout` function, along with the numerical 
            value assigned to the `wait` argument in our `debounce` method. 
            If no value is passed to the `wait` argument in our `debounce` 
            method, pass a value of 200 milliseconds to the `setTimeout` 
            function.  
        */
        timeout = setTimeout(later, wait || 200);

        /*
            Typically, you want the function passed in the `func` argument
            of our `debounce` method to execute once *after* the `wait` 
            period has been met for the event that our `debounce` method is 
            bound to (the trailing side). However, if you want the function 
            to execute once *before* the event has finished (on the leading 
            side), you can pass `true` in the `immediate` argument of our 
            `debounce` method.

            If `true` is passed in the `immediate` argument of our 
            `debounce` method, the value assigned to the `callNow` variable 
            declared above will be `true` only after the *first* time the 
            event that our `debounce` method is bound to has fired.

            After the first time the event is fired, the `timeout` variable
            will contain a falsey value. Therfore, the result of the 
            expression that gets assigned to the `callNow` variable is 
            `true` and the function passed in the `func` argument of our
            `debounce` method is exected in the line of code below.

            Every subsequent time the event that our `debounce` method is 
            bound to fires within the `wait` period, the `timeout` variable 
            holds the numerical ID returned from the `setTimout` function 
            assigned to it when the previous event was fired, and the 
            `debounce` method was executed.

            This means that for all subsequent events within the `wait`
            period, the `timeout` variable holds a truthy value, and the
            result of the expression that gets assigned to the `callNow`
            variable is `false`. Therefore, the function passed in the 
            `func` argument of our `debounce` method will not be executed.  

            Lastly, when the `wait` period is met and the `later` function
            that is passed in the `setTimeout` function executes, the 
            result is that it just assigns `null` to the `timeout` 
            variable. The `func` argument passed in our `debounce` method 
            will not be executed because the `if` condition inside the 
            `later` function fails. 
        */
        if ( callNow ) { 
            func.apply(context, args);
        }
    };
};

we're all using Promises now

Many implementations I've seen over-complicate the problem or have other hygiene issues. It's 2021 and we've been using Promises for a long time now – and for good reason, too. Promises clean up asynchronous programs and reduce the opportunities for mistakes to happen. In this post we will write our own debounce. This implementation will -

• have at most one promise pending at any given time (per debounced task)
• stop memory leaks by properly cancelling pending promises
• resolve only the latest promise
• demonstrate proper behaviour with live code demos

We write debounce with its two parameters, the task to debounce, and the amount of milliseconds to delay, ms. We introduce a single local binding for its local state, t -

function debounce (task, ms) {
  let t = { promise: null, cancel: _ => void 0 }
  return async (...args) => {
    try {
      t.cancel()
      t = deferred(ms)
      await t.promise
      await task(...args)
    }
    catch (_) { /* prevent memory leak */ }
  }
}

We depend on a reusable deferred function, which creates a new promise that resolves in ms milliseconds. It introduces two local bindings, the promise itself, an the ability to cancel it -

function deferred (ms) {
  let cancel, promise = new Promise((resolve, reject) => {
    cancel = reject
    setTimeout(resolve, ms)
  })
  return { promise, cancel }
}

click counter example

In this first example, we have a button that counts the user's clicks. The event listener is attached using debounce, so the counter is only incremented after a specified duration -

// debounce, deferred
function debounce (task, ms) { let t = { promise: null, cancel: _ => void 0 }; return async (...args) => { try { t.cancel(); t = deferred(ms); await t.promise; await task(...args); } catch (_) { console.log("cleaning up cancelled promise") } } }
function deferred (ms) { let cancel, promise = new Promise((resolve, reject) => { cancel = reject; setTimeout(resolve, ms) }); return { promise, cancel } }

// dom references
const myform = document.forms.myform
const mycounter = myform.mycounter

// event handler
function clickCounter (event) {
  mycounter.value = Number(mycounter.value) + 1
}

// debounced listener
myform.myclicker.addEventListener("click", debounce(clickCounter, 1000))

<form id="myform">
<input name="myclicker" type="button" value="click" />
<output name="mycounter">0</output>
</form>

live query example, "autocomplete"

In this second example, we have a form with a text input. Our search query is attached using debounce -

// debounce, deferred
function debounce (task, ms) { let t = { promise: null, cancel: _ => void 0 }; return async (...args) => { try { t.cancel(); t = deferred(ms); await t.promise; await task(...args); } catch (_) { console.log("cleaning up cancelled promise") } } }
function deferred (ms) { let cancel, promise = new Promise((resolve, reject) => { cancel = reject; setTimeout(resolve, ms) }); return { promise, cancel } }

// dom references
const myform = document.forms.myform
const myresult = myform.myresult

// event handler
function search (event) {
  myresult.value = `Searching for: ${event.target.value}`
}

// debounced listener
myform.myquery.addEventListener("keypress", debounce(search, 1000))

<form id="myform">
<input name="myquery" placeholder="Enter a query..." />
<output name="myresult"></output>
</form>

What you want to do is the following: If you try to call a function right after another, the first should be cancelled and the new one should wait for a given timeout and then execute. So in effect you need some way of cancelling the timeout of the first function? But how? You could call the function, and pass the returning timeout-id and then pass that ID into any new functions. But the solution above is way more elegant.

What it does is effectively make the timeout variable available in the scope of returned function. So when a 'resize' event is fired it does not call debounce() again, hence the timeout content is not changed (!) and still available for the "next function call".

The key thing here is basically that we call the internal function every time we have a resize event. Perhaps it is more clear if we imagine all resize-events is in an array:

var events = ['resize', 'resize', 'resize'];
var timeout = null;
for (var i = 0; i < events.length; i++){
    if (immediate && !timeout) func.apply(this, arguments);
    clearTimeout(timeout); // does not do anything if timeout is null.
    timeout = setTimeout(function(){
        timeout = null;
        if (!immediate) func.apply(this, arguments);
    }
}

You see the timeout is available to the next iteration? And there is no reason, in my opinion to rename this to content and arguments to args.