[javascript] JavaScript window resize event

How can I hook into a browser window resize event?

There's a jQuery way of listening for resize events but I would prefer not to bring it into my project for just this one requirement.

First off, I know the addEventListener method has been mentioned in the comments above, but I didn't see any code. Since it's the preferred approach, here it is:

window.addEventListener('resize', function(event){
  // do stuff here
});

Here's a working sample.

<script language="javascript">
    window.onresize = function() {
    document.getElementById('ctl00_ContentPlaceHolder1_Accordion1').style.height = '100%';
} 

</script>

Solution for 2018+:

You should use ResizeObserver. It is a browser-native solution that has a much better performance than to use the resize event. In addition, it not only supports the event on the document but also on arbitrary elements.

var ro = new ResizeObserver( entries => {
  for (let entry of entries) {
    const cr = entry.contentRect;
    console.log('Element:', entry.target);
    console.log(`Element size: ${cr.width}px x ${cr.height}px`);
    console.log(`Element padding: ${cr.top}px ; ${cr.left}px`);
  }
});

// Observe one or multiple elements
ro.observe(someElement);

Currently, Firefox, Chrome, Safari, and Edge support it. For other (and older) browsers you have to use a polyfill.

The following blog post may be useful to you: Fixing the window resize event in IE

It provides this code:

Sys.Application.add_load(function(sender, args) {
    $addHandler(window, 'resize', window_resize);
});

var resizeTimeoutId;

function window_resize(e) {
     window.clearTimeout(resizeTimeoutId);
     resizeTimeoutId = window.setTimeout('doResizeCode();', 10);
}

window.onresize = function() {
    // your code
};

The resize event should never be used directly as it is fired continuously as we resize.

Use a debounce function to mitigate the excess calls.

window.addEventListener('resize',debounce(handler, delay, immediate),false);

Here's a common debounce floating around the net, though do look for more advanced ones as featuerd in lodash.

const debounce = (func, wait, immediate) => {
    var timeout;
    return () => {
        const context = this, args = arguments;
        const later = function() {
            timeout = null;
            if (!immediate) func.apply(context, args);
        };
        const callNow = immediate && !timeout;
        clearTimeout(timeout);
        timeout = setTimeout(later, wait);
        if (callNow) func.apply(context, args);
    };
};

This can be used like so...

window.addEventListener('resize', debounce(() => console.log('hello'),
200, false), false);

It will never fire more than once every 200ms.

For mobile orientation changes use:

window.addEventListener('orientationchange', () => console.log('hello'), false);

Here's a small library I put together to take care of this neatly.

The already mentioned solutions above will work if all you want to do is resize the window and window only. However, if you want to have the resize propagated to child elements, you will need to propagate the event yourself. Here's some example code to do it:

window.addEventListener("resize", function () {
  var recResizeElement = function (root) {
    Array.prototype.forEach.call(root.childNodes, function (el) {

      var resizeEvent = document.createEvent("HTMLEvents");
      resizeEvent.initEvent("resize", false, true);
      var propagate = el.dispatchEvent(resizeEvent);

      if (propagate)
        recResizeElement(el);
    });
  };
  recResizeElement(document.body);
});

Note that a child element can call

 event.preventDefault();

on the event object that is passed in as the first Arg of the resize event. For example:

var child1 = document.getElementById("child1");
child1.addEventListener("resize", function (event) {
  ...
  event.preventDefault();
});

Never override the window.onresize function.

Instead, create a function to add an Event Listener to the object or element. This checks and incase the listeners don't work, then it overrides the object's function as a last resort. This is the preferred method used in libraries such as jQuery.

object: the element or window object
type: resize, scroll (event type)
callback: the function reference

var addEvent = function(object, type, callback) {
    if (object == null || typeof(object) == 'undefined') return;
    if (object.addEventListener) {
        object.addEventListener(type, callback, false);
    } else if (object.attachEvent) {
        object.attachEvent("on" + type, callback);
    } else {
        object["on"+type] = callback;
    }
};

Then use is like this:

addEvent(window, "resize", function_reference);

or with an anonymous function:

addEvent(window, "resize", function(event) {
  console.log('resized');
});

I do believe that the correct answer has already been provided by @Alex V, yet the answer does require some modernization as it is over five years old now.

There are two main issues:

1. Never use object as a parameter name. It is a reservered word. With this being said, @Alex V's provided function will not work in strict mode.

2. The addEvent function provided by @Alex V does not return the event object if the addEventListener method is used. Another parameter should be added to the addEvent function to allow for this.

NOTE: The new parameter to addEvent has been made optional so that migrating to this new function version will not break any previous calls to this function. All legacy uses will be supported.

Here is the updated addEvent function with these changes:

/*
    function: addEvent

    @param: obj         (Object)(Required)

        -   The object which you wish
            to attach your event to.

    @param: type        (String)(Required)

        -   The type of event you
            wish to establish.

    @param: callback    (Function)(Required)

        -   The method you wish
            to be called by your
            event listener.

    @param: eventReturn (Boolean)(Optional)

        -   Whether you want the
            event object returned
            to your callback method.
*/
var addEvent = function(obj, type, callback, eventReturn)
{
    if(obj == null || typeof obj === 'undefined')
        return;

    if(obj.addEventListener)
        obj.addEventListener(type, callback, eventReturn ? true : false);
    else if(obj.attachEvent)
        obj.attachEvent("on" + type, callback);
    else
        obj["on" + type] = callback;
};

An example call to the new addEvent function:

var watch = function(evt)
{
    /*
        Older browser versions may return evt.srcElement
        Newer browser versions should return evt.currentTarget
    */
    var dimensions = {
        height: (evt.srcElement || evt.currentTarget).innerHeight,
        width: (evt.srcElement || evt.currentTarget).innerWidth
    };
};

addEvent(window, 'resize', watch, true);

You can use following approach which is ok for small projects

<body onresize="yourHandler(event)">

function yourHandler(e) {
  console.log('Resized:', e.target.innerWidth)
}

<body onresize="yourHandler(event)">
  Content... (resize browser to see)
</body>

Thanks for referencing my blog post at http://mbccs.blogspot.com/2007/11/fixing-window-resize-event-in-ie.html.

While you can just hook up to the standard window resize event, you'll find that in IE, the event is fired once for every X and once for every Y axis movement, resulting in a ton of events being fired which might have a performance impact on your site if rendering is an intensive task.

My method involves a short timeout that gets cancelled on subsequent events so that the event doesn't get bubbled up to your code until the user has finished resizing the window.

var EM = new events_managment();

EM.addEvent(window, 'resize', function(win,doc, event_){
    console.log('resized');
    //EM.removeEvent(win,doc, event_);
});

function events_managment(){
    this.events = {};
    this.addEvent = function(node, event_, func){
        if(node.addEventListener){
            if(event_ in this.events){
                node.addEventListener(event_, function(){
                    func(node, event_);
                    this.events[event_](win_doc, event_);
                }, true);
            }else{
                node.addEventListener(event_, function(){
                    func(node, event_);
                }, true);
            }
            this.events[event_] = func;
        }else if(node.attachEvent){

            var ie_event = 'on' + event_;
            if(ie_event in this.events){
                node.attachEvent(ie_event, function(){
                    func(node, ie_event);
                    this.events[ie_event]();
                });
            }else{
                node.attachEvent(ie_event, function(){
                    func(node, ie_event);
                });
            }
            this.events[ie_event] = func;
        }
    }
    this.removeEvent = function(node, event_){
        if(node.removeEventListener){
            node.removeEventListener(event_, this.events[event_], true);
            this.events[event_] = null;
            delete this.events[event_];
        }else if(node.detachEvent){
            node.detachEvent(event_, this.events[event_]);
            this.events[event_] = null;
            delete this.events[event_];
        }
    }
}