CODEWITHSUNDEEP
javascripteventsdom
Navneet
Navneet

Reputation: 11887

How to find event listeners on a DOM node in JavaScript or in debugging?

I have a page where some event listeners are attached to input boxes and select boxes. Is there a way to find out which event listeners are observing a particular DOM node and for what event?

Events are attached using:

  1. Prototype's Event.observe;
  2. DOM's addEventListener;
  3. As element attribute element.onclick.

Upvotes: 1149

Views: 1010938

Answers (22)

Jan Turoň
Jan Turoň

Reputation: 32922

1: Prototype.observe uses Element.addEventListener (see the source code)

2: You can override Element.addEventListener to remember the added listeners (handy property EventListenerList was removed from DOM3 spec proposal). Run this code before any event is attached:

  Element.prototype._addEventListener = Element.prototype.addEventListener;
  Element.prototype.addEventListener = function(a,b,c) {
    this._addEventListener(a,b,c);
    if(!this.eventListenerList) this.eventListenerList = {};
    if(!this.eventListenerList[a]) this.eventListenerList[a] = [];
    this.eventListenerList[a].push(b);
  };

Read all the events by:

var clicks = someElement.eventListenerList.click;
if(clicks) clicks.forEach(function(f) {
  alert("I listen to this function: "+f.toString());
});

And don't forget to override Element.removeEventListener to remove the event from the custom Element.eventListenerList.

3: the Element.onclick property needs special care here:

if(someElement.onclick)
  alert("I also listen tho this: "+someElement.onclick.toString());

4: don't forget the Element.onclick content attribute: these are two different things:

someElement.onclick = someHandler; // IDL attribute
someElement.setAttribute("onclick","otherHandler(event)"); // content attribute

So you need to handle it, too:

var click = someElement.getAttribute("onclick");
if(click) alert("I even listen to this: "+click);

The Visual Event bookmarklet (mentioned in the most popular answer) only steals the custom library handler cache:

It turns out that there is no standard method provided by the W3C recommended DOM interface to find out what event listeners are attached to a particular element. While this may appear to be an oversight, there was a proposal to include a property called eventListenerList to the level 3 DOM specification, but was unfortunately been removed in later drafts. As such we are forced to looked at the individual Javascript libraries, which typically maintain a cache of attached events (so they can later be removed and perform other useful abstractions).

As such, in order for Visual Event to show events, it must be able to parse the event information out of a Javascript library.

Element overriding may be questionable (i.e. because there are some DOM specific features like live collections, which can not be coded in JS), but it gives the eventListenerList support natively and it works in Chrome, Firefox and Opera (doesn't work in IE7).

Upvotes: 38

MoonLite
MoonLite

Reputation: 5301

For quickly listing and/or removing all event handlers on document or any element:

(only works in Chrome console because it uses getEventListeners(), see:
https://developer.chrome.com/docs/devtools/console/utilities/#getEventListeners-function
)

let list = [];
let targets = Array.from(document.querySelectorAll('*'));
targets.push(document);
targets.forEach((obj) => {
    let listeners = getEventListeners(obj);
    for (p in listeners) {
        listeners[p].forEach((l) => {
            removeEventListener(l.type, l.listener, l.useCapture);
            list.push([obj, l.type, l.listener, '' + l.listener, l.useCapture]);
        });
    }
});
console.table(list);

Or type in console (or code):

debugger;

to step through code to find what exactly happens when e.g. hovering over element.

debugger; also works great to halt halt any runaway loops.

Slightly related, to stop most loops (hacky, but works):

for (i = 0; i < 10000; i++) {
  clearTimeout(i);
  clearInterval(i);
  cancelAnimationFrame(i);
}

Upvotes: 5

Akaisteph7
Akaisteph7

Reputation: 6524

Currently, Sep. 2023 in Firefox Browser 117.0.1, you can see this by clicking on the appropriate "event" button in the "Inspector" in the "Developer Tools" as shown below:

enter image description here

Upvotes: 4

akinuri
akinuri

Reputation: 12027

I was recently working with events and wanted to view/control all events in a page. Having looked at possible solutions, I've decided to go my own way and create a custom system to monitor events. So, I did three things.

First, I needed a container for all the event listeners in the page: that's theEventListeners object. It has three useful methods: add(), remove(), and get().

Next, I created an EventListener object to hold the necessary information for the event, i.e.: target, type, callback, options, useCapture, wantsUntrusted, and added a method remove() to remove the listener.

Lastly, I extended the native addEventListener() and removeEventListener() methods to make them work with the objects I've created (EventListener and EventListeners).

Usage:

var bodyClickEvent = document.body.addEventListener("click", function () {
    console.log("body click");
});

// bodyClickEvent.remove();

addEventListener() creates an EventListener object, adds it to EventListeners and returns the EventListener object, so it can be removed later.

EventListeners.get() can be used to view the listeners in the page. It accepts an EventTarget or a string (event type).

// EventListeners.get(document.body);
// EventListeners.get("click");

Demo

Let's say we want to know every event listener in this current page. We can do that (assuming you're using a script manager extension, Tampermonkey in this case). Following script does this:

// ==UserScript==
// @name         New Userscript
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @include      https://stackoverflow.com/*
// @grant        none
// ==/UserScript==

(function() {
    fetch("https://raw.githubusercontent.com/akinuri/js-lib/master/event/event-listeners/event-listeners.js")
        .then(function (response) {
            return response.text();
        })
        .then(function (text) {
            eval(text);
            window.EventListeners = EventListeners;
        });
})(window);

And when we list all the listeners, it says there are 299 event listeners. There "seems" to be some duplicates, but I don't know if they're really duplicates. Not every event type is duplicated, so all those "duplicates" might be an individual listener.

screenshot of console listing all event listeners in this page

Code can be found at my repository. I didn't want to post it here because it's rather long.

Update: This doesn't seem to work with jQuery. When I examine the EventListener, I see that the callback is

function(b){return"undefined"!=typeof r&&r.event.triggered!==b.type?r.event.dispatch.apply(a,arguments):void 0}

I believe this belongs to jQuery, and is not the actual callback. jQuery stores the actual callback in the properties of the EventTarget:

$(document.body).click(function () {
    console.log("jquery click");
});

enter image description here

To remove an event listener, the actual callback needs to be passed to the removeEventListener() method. So in order to make this work with jQuery, it needs further modification. I might fix that in the future.

Upvotes: 5

Raghav
Raghav

Reputation: 7523

Chrome, Vivaldi and Safari support getEventListeners(domElement) in their Developer Tools console.

For majority of the debugging purposes, this could be used.

Below is a very good reference to use it:
getEventListeners function

Highly voted tip from Clifford Fajardo from the comments:

getEventListeners($0) will get the event listeners for the element you have focused on in the Chrome dev tools.

Upvotes: 684

aljgom
aljgom

Reputation: 9189

Update 2022:

In the Chrome Developer Tools, in the Elements panel, there is the Event Listeners tab, where you can see listeners for the element.

You can also unselect "Ancestors" so it only shows the listeners for that element

Developer Tools

Upvotes: 17

Gabriel Petersson
Gabriel Petersson

Reputation: 10482

Paste in console to get all eventListeners printed beside their HTML element

Array.from(document.querySelectorAll("*")).forEach(element => {
    const events = getEventListeners(element)
    if (Object.keys(events).length !== 0) {
         console.log(element, events)
    }
})

Upvotes: 26

Crescent Fresh
Crescent Fresh

Reputation: 117008

It depends on how the events are attached. For illustration presume we have the following click handler:

var handler = function() { alert('clicked!') };

We're going to attach it to our element using different methods, some which allow inspection and some that don't.

Method A) single event handler

element.onclick = handler;
// inspect
console.log(element.onclick); // "function() { alert('clicked!') }"

Method B) multiple event handlers

if(element.addEventListener) { // DOM standard
    element.addEventListener('click', handler, false)
} else if(element.attachEvent) { // IE
    element.attachEvent('onclick', handler)
}
// cannot inspect element to find handlers

Method C): jQuery

$(element).click(handler);

  • 1.3.x

     // inspect
 var clickEvents = $(element).data("events").click;
 jQuery.each(clickEvents, function(key, value) {
     console.log(value) // "function() { alert('clicked!') }"
 })

  • 1.4.x (stores the handler inside an object)

     // inspect
 var clickEvents = $(element).data("events").click;
 jQuery.each(clickEvents, function(key, handlerObj) {
     console.log(handlerObj.handler) // "function() { alert('clicked!') }"
     // also available: handlerObj.type, handlerObj.namespace
 })

  • 1.7+ (very nice)

    Made using knowledge from this comment.

     events = $._data(this, 'events');
 for (type in events) {
   events[type].forEach(function (event) {
     console.log(event['handler']);
   });
 }

(See jQuery.fn.data and jQuery.data)

Method D): Prototype (messy)

$(element).observe('click', handler);

  • 1.5.x

     // inspect
 Event.observers.each(function(item) {
     if(item[0] == element) {
         console.log(item[2]) // "function() { alert('clicked!') }"
     }
 })

  • 1.6 to 1.6.0.3, inclusive (got very difficult here)

     // inspect. "_eventId" is for < 1.6.0.3 while 
 // "_prototypeEventID" was introduced in 1.6.0.3
 var clickEvents = Event.cache[element._eventId || (element._prototypeEventID || [])[0]].click;
 clickEvents.each(function(wrapper){
     console.log(wrapper.handler) // "function() { alert('clicked!') }"
 })

  • 1.6.1 (little better)

     // inspect
 var clickEvents = element.getStorage().get('prototype_event_registry').get('click');
 clickEvents.each(function(wrapper){
     console.log(wrapper.handler) // "function() { alert('clicked!') }"
 })

When clicking the resulting output in the console (which shows the text of the function), the console will navigate directly to the line of the function's declaration in the relevant JS file.

Upvotes: 402

Andrew Hedges
Andrew Hedges

Reputation: 21796

If you just need to inspect what's happening on a page, you might try the Visual Event bookmarklet.

Update: Visual Event 2 available.

Upvotes: 542

Aiden Waterman
Aiden Waterman

Reputation: 63

changing these functions will allow you to log the listeners added:

EventTarget.prototype.addEventListener
EventTarget.prototype.attachEvent
EventTarget.prototype.removeEventListener
EventTarget.prototype.detachEvent

read the rest of the listeners with

console.log(someElement.onclick);
console.log(someElement.getAttribute("onclick"));

Upvotes: 2

Pranay Soni
Pranay Soni

Reputation: 1066

Use getEventListeners in Google Chrome:

getEventListeners(document.getElementByID('btnlogin'));
getEventListeners($('#btnlogin'));

Upvotes: 63

T.Todua
T.Todua

Reputation: 56537

There exists nice jQuery Events extension :

enter image description here (topic source)

Upvotes: 3

n00b
n00b

Reputation: 5732

Fully working solution based on answer by Jan Turon - behaves like getEventListeners() from console:

(There is a little bug with duplicates. It doesn't break much anyway.)

(function() {
  Element.prototype._addEventListener = Element.prototype.addEventListener;
  Element.prototype.addEventListener = function(a,b,c) {
    if(c==undefined)
      c=false;
    this._addEventListener(a,b,c);
    if(!this.eventListenerList)
      this.eventListenerList = {};
    if(!this.eventListenerList[a])
      this.eventListenerList[a] = [];
    //this.removeEventListener(a,b,c); // TODO - handle duplicates..
    this.eventListenerList[a].push({listener:b,useCapture:c});
  };

  Element.prototype.getEventListeners = function(a){
    if(!this.eventListenerList)
      this.eventListenerList = {};
    if(a==undefined)
      return this.eventListenerList;
    return this.eventListenerList[a];
  };
  Element.prototype.clearEventListeners = function(a){
    if(!this.eventListenerList)
      this.eventListenerList = {};
    if(a==undefined){
      for(var x in (this.getEventListeners())) this.clearEventListeners(x);
        return;
    }
    var el = this.getEventListeners(a);
    if(el==undefined)
      return;
    for(var i = el.length - 1; i >= 0; --i) {
      var ev = el[i];
      this.removeEventListener(a, ev.listener, ev.useCapture);
    }
  };

  Element.prototype._removeEventListener = Element.prototype.removeEventListener;
  Element.prototype.removeEventListener = function(a,b,c) {
    if(c==undefined)
      c=false;
    this._removeEventListener(a,b,c);
      if(!this.eventListenerList)
        this.eventListenerList = {};
      if(!this.eventListenerList[a])
        this.eventListenerList[a] = [];

      // Find the event in the list
      for(var i=0;i<this.eventListenerList[a].length;i++){
          if(this.eventListenerList[a][i].listener==b, this.eventListenerList[a][i].useCapture==c){ // Hmm..
              this.eventListenerList[a].splice(i, 1);
              break;
          }
      }
    if(this.eventListenerList[a].length==0)
      delete this.eventListenerList[a];
  };
})();

Usage:

someElement.getEventListeners([name]) - return list of event listeners, if name is set return array of listeners for that event

someElement.clearEventListeners([name]) - remove all event listeners, if name is set only remove listeners for that event

Upvotes: 13

simonzack
simonzack

Reputation: 20938

The Firefox developer tools now does this. Events are shown by clicking the "ev" button on the right of each element's display, including jQuery and DOM events.

Screenshot of Firefox developer tools' event listener button in the inspector tab

Upvotes: 20

Michael Butler
Michael Butler

Reputation: 6309

If you have Firebug, you can use console.dir(object or array) to print a nice tree in the console log of any JavaScript scalar, array, or object.

Try: 

console.dir(clickEvents);

or

console.dir(window);

Upvotes: 13

jrz
jrz

Reputation: 4582

You could wrap the native DOM methods for managing event listeners by putting this at the top of your <head>:

<script>
    (function(w){
        var originalAdd = w.addEventListener;
        w.addEventListener = function(){
            // add your own stuff here to debug
            return originalAdd.apply(this, arguments);
        };

        var originalRemove = w.removeEventListener;
        w.removeEventListener = function(){
            // add your own stuff here to debug
            return originalRemove.apply(this, arguments);
        };
    })(window);
</script>

H/T @les2

Upvotes: 25

Joel Barba
Joel Barba

Reputation: 113

I am trying to do that in jQuery 2.1, and with the "$().click() -> $(element).data("events").click;" method it doesn't work.

I realized that only the $._data() functions works in my case :

	$(document).ready(function(){

		var node = $('body');
		
        // Bind 3 events to body click
		node.click(function(e) { alert('hello');  })
			.click(function(e) { alert('bye');  })
			.click(fun_1);

        // Inspect the events of body
		var events = $._data(node[0], "events").click;
		var ev1 = events[0].handler // -> function(e) { alert('hello')
		var ev2 = events[1].handler // -> function(e) { alert('bye')
		var ev3 = events[2].handler // -> function fun_1()
        
		$('body')
			.append('<p> Event1 = ' + eval(ev1).toString() + '</p>')
			.append('<p> Event2 = ' + eval(ev2).toString() + '</p>')
			.append('<p> Event3 = ' + eval(ev3).toString() + '</p>');        
	
	});

	function fun_1() {
		var txt = 'text del missatge';	 
		alert(txt);
	}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<body>
</body>

Upvotes: 5

Luke
Luke

Reputation: 20071

(Rewriting the answer from this question since it's relevant here.)

When debugging, if you just want to see the events, I recommend either...

  1. Visual Event
  2. The Elements section of Chrome's Developer Tools: select an element and look for "Event Listeners" on the bottom right (similar in Firefox)

If you want to use the events in your code, and you are using jQuery before version 1.8, you can use: 

$(selector).data("events")

to get the events. As of version 1.8, using .data("events") is discontinued (see this bug ticket). You can use:

$._data(element, "events")

Another example: Write all click events on a certain link to the console:

var $myLink = $('a.myClass');
console.log($._data($myLink[0], "events").click);

(see http://jsfiddle.net/HmsQC/ for a working example)

Unfortunately, using $._data this is not recommended except for debugging since it is an internal jQuery structure, and could change in future releases. Unfortunately I know of no other easy means of accessing the events.

Upvotes: 46

Daniel Sokolowski
Daniel Sokolowski

Reputation: 12488

Opera 12 (not the latest Chrome Webkit engine based) Dragonfly has had this for a while and is obviously displayed in the DOM structure. In my opinion it is a superior debugger and is the only reason remaining why I still use the Opera 12 based version (there is no v13, v14 version and the v15 Webkit based lacks Dragonfly still)

enter image description here

Upvotes: 10

Ronnie
Ronnie

Reputation: 393

Prototype 1.7.1 way

function get_element_registry(element) {
    var cache = Event.cache;
    if(element === window) return 0;
    if(typeof element._prototypeUID === 'undefined') {
        element._prototypeUID = Element.Storage.UID++;
    }
    var uid =  element._prototypeUID;           
    if(!cache[uid]) cache[uid] = {element: element};
    return cache[uid];
}

Upvotes: 5

Ivan Castellanos
Ivan Castellanos

Reputation: 8251

It is possible to list all event listeners in JavaScript: It's not that hard; you just have to hack the prototype's method of the HTML elements (before adding the listeners).

function reportIn(e){
    var a = this.lastListenerInfo[this.lastListenerInfo.length-1];
    console.log(a)
}


HTMLAnchorElement.prototype.realAddEventListener = HTMLAnchorElement.prototype.addEventListener;

HTMLAnchorElement.prototype.addEventListener = function(a,b,c){
    this.realAddEventListener(a,reportIn,c); 
    this.realAddEventListener(a,b,c); 
    if(!this.lastListenerInfo){  this.lastListenerInfo = new Array()};
    this.lastListenerInfo.push({a : a, b : b , c : c});
};

Now every anchor element (a) will have a lastListenerInfo property wich contains all of its listeners. And it even works for removing listeners with anonymous functions.

Upvotes: 94

Ishan
Ishan

Reputation: 1376

WebKit Inspector in Chrome or Safari browsers now does this. It will display the event listeners for a DOM element when you select it in the Elements pane.

Upvotes: 99

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