Is there a way to find the event handlers of an element with Javascript?

Question

Say you have this div:

<div id="foo">bar</div>

And you do this:

$("#foo").click(someFunction);

somewhere inside your Javascript code.

Once the page has been loaded, is there a way to find out, via firebug or inspect element in Chrome, or anything else, that the click event for #foo is bound to someFunction? I.e., find this out without having to look through all your code?

Answer 1

Yes it is possible, you can even do it in plain-JS, but only if you intercept/override the addEventListener and removeEventListener prototype functions with your own interceptor, so you can intercept them.

This will work for anything that has been added with addEventListener (and it accounts for removeEventListener).

But if you added them without EventListener, e.g. with element.onclick (or in the onclick/onAnything-attribute in the markup), this won't list them, you'll have to manually check for them.

Be sure that the bellow JavaScript is the first script that is executed on your page, otherwise it might not work properly.

Here's how (TypeScript):

type EventHandlerMapType = {
    // [key: EventTarget]: { [type: string]: EventListenerOrEventListenerObject[] };
    [key: string]: { [type: string]: EventListenerOrEventListenerObject[] };
};


type EventHandlerMapValue = { [type: string]: EventListenerOrEventListenerObject[] };
interface EventTarget
{
    getEventHandlers: (type?: string) => EventHandlerMapValue | EventListenerOrEventListenerObject[];
}





// function addEventListener<K extends keyof ElementEventMap>(type: K, listener: (this: Element, ev: ElementEventMap[K]) => any, options ?: boolean | AddEventListenerOptions): void;
// addEventListener(type: string, listener: EventListenerOrEventListenerObject, options ?: boolean | AddEventListenerOptions): void;


(function ()
{
    // Store the handlers by element reference
    // WeakMap can take an object, such as an Element, as a key, object cannot. 
    // This is useful because WeakMap allows for garbage collection of the keys(the elements), 
    // meaning when an Element is removed from the DOM and no longer referenced, it gets garbage - collected, 
    // and its entry in the WeakMap is automatically removed. 
    // This prevents memory leaks.
    const eventHandlerMap = new WeakMap<EventTarget>(); // Dictionary<Element, { type:[]}> // where type is string and array is an array of handlers/listeners

    // Override the native addEventListener
    const originalAddEventListener = EventTarget.prototype.addEventListener;

    
    
    EventTarget.prototype.addEventListener = function (type: string, listener: EventListenerOrEventListenerObject, options?: boolean | AddEventListenerOptions)
    {
        // Call the original addEventListener to ensure normal behavior
        originalAddEventListener.call(this, type, listener, options);

        // Initialize tracking for the current element if it doesn't exist
        if (!eventHandlerMap.has(this))
        {
            eventHandlerMap.set(this, {});
        }

        // Get the event type handlers for this element
        const handlersForElement = eventHandlerMap.get(this);
        if (!handlersForElement[type])
        {
            handlersForElement[type] = [];
        }

        // Add the handler to the list for this event type
        handlersForElement[type].push(listener);
    };

    // Override the native removeEventListener
    const originalRemoveEventListener = EventTarget.prototype.removeEventListener;
    EventTarget.prototype.removeEventListener = function (type: string, listener: EventListenerOrEventListenerObject, options?: boolean | AddEventListenerOptions)
    {
        // Call the original removeEventListener to ensure normal behavior
        originalRemoveEventListener.call(this, type, listener, options);

        // Remove the handler from the tracking list
        if (eventHandlerMap.has(this))
        {
            const handlersForElement = eventHandlerMap.get(this);
            if (handlersForElement[type])
            {
                // Filter out the handler that matches the one being removed
                handlersForElement[type] = handlersForElement[type].filter((h: EventListenerOrEventListenerObject) => h !== listener);

                // Clean up if no handlers left for this event type
                if (handlersForElement[type].length === 0)
                {
                    delete handlersForElement[type];
                }
            }

            // Clean up the element if no handlers left for any event type
            if (Object.keys(handlersForElement).length === 0)
            {
                eventHandlerMap.delete(this);
            }
        }
    };

    // Function to retrieve all event handlers for an element
    EventTarget.prototype.getEventHandlers = function (type?: string): EventHandlerMapValue | EventListenerOrEventListenerObject[]
    {
        // Get the tracking list for the current element
        const handlersForElement = eventHandlerMap.get(this) || {};

        if (type)
        {
            // If a specific event type is requested, return its handlers
            return handlersForElement[type] || [];
        }

        // If no type is specified, return all handlers grouped by type
        return handlersForElement;
    };

})();

Now on EventTarget (Element, Node, etc):

getEventHandlers(type?: string)

or in plain-JS

(function () {
    var eventHandlerMap = new WeakMap();
    var originalAddEventListener = EventTarget.prototype.addEventListener;
    EventTarget.prototype.addEventListener = function (type, listener, options) {
        originalAddEventListener.call(this, type, listener, options);
        if (!eventHandlerMap.has(this)) {
            eventHandlerMap.set(this, {});
        }
        var handlersForElement = eventHandlerMap.get(this);
        if (!handlersForElement[type]) {
            handlersForElement[type] = [];
        }
        handlersForElement[type].push(listener);
    };
    var originalRemoveEventListener = EventTarget.prototype.removeEventListener;
    EventTarget.prototype.removeEventListener = function (type, listener, options) {
        originalRemoveEventListener.call(this, type, listener, options);
        if (eventHandlerMap.has(this)) {
            var handlersForElement = eventHandlerMap.get(this);
            if (handlersForElement[type]) {
                handlersForElement[type] = handlersForElement[type].filter(function (h) { return h !== listener; });
                if (handlersForElement[type].length === 0) {
                    delete handlersForElement[type];
                }
            }
            if (Object.keys(handlersForElement).length === 0) {
                eventHandlerMap.delete(this);
            }
        }
    };
    EventTarget.prototype.getEventHandlers = function (type) {
        var handlersForElement = eventHandlerMap.get(this) || {};
        if (type) {
            return handlersForElement[type] || [];
        }
        return handlersForElement;
    };
})();

Tests:

var btnCreated = document.createElement("button");
btnCreated.textContent = "Hello Kitty";
btnCreated.value = "Hello Kitty";
document.body.appendChild(btnCreated);
var btn = document.querySelector('button');
function handleClick() {
    console.log('Button clicked');
}
btn.addEventListener('click', handleClick);
btn.addEventListener('clock', handleClick);
console.log(btn.getEventHandlers('click'));
console.log("before click");
btn.click();
console.log("after click");
btn.removeEventListener('click', handleClick);
console.log("before click after click removed");
btn.click();
console.log("after click after click removed");
console.log("click handlers", btn.getEventHandlers('click'));
console.log("all handlers", btn.getEventHandlers());

Answer 2

You can use the console in browsers built-in developer tools. They are built-in for IE and Chrome while FF will need the Firebug AddOn installed. I don't know about other browsers.

Using the debugger console you can use jQuery data("events") to query for the attached events of an element. In addition those consoles also let you dynamically drill down into any additional detail of the events you are interested in.

$("#foo").data("events");

Executing the above in the console will display an object with a property for each event it found. In your example it returns an object with click property of type array storing all click events.

If you have click events and you want just that object you can execute the following in the console:

$("#foo").data("events").click;

Each event object has a handler property which will show you the function they are bound to:

Object
data: null
guid: 2
handler: function mytestFunction(){
arguments: null
caller: null
guid: 2
length: 0
name: "mytestFunction"
prototype: mytestFunction
__proto__: function Empty() {}
namespace: ""
origType: "click"
quick: null
selector: null
type: "click"
__proto__: Object

See DEMO, showing how to query and display the objects in the console.

Alternatively you can also use the "handlers" for an overall summary object:

$("#foo").data("handlers");

Note though that .data("events/handlers") will not include any events wired up embedded in html such as this:

<div id="foo" onclick="...">bar</div>

More information on the data() are in the documentation

Answer 3

I would suggest using Visual Event 2.

Here is the description on how to use it. I use it almost everyday and its a very good tool.

Answer 4

Chrome Developer tools does this.

1. right click an element
2. click inspect element
3. in the right hand column, scroll down to event listeners

This will give you a list of event listeners on the object that can be expanded to find their source and the attached function.

Firebug has this information under the DOM tab, but it's less user friendly.

Answer 5

I don't know about Firefox, but there's an easy way to see event listeners in Chrome and Safari. Just open the Developer Tools, select your element and scroll to the bottom of the CSS properties panel. You'll find an "Event Listeners" section.