How to check whether dynamically attached event listener exists or not

Question

Here is my problem  is it somehow possible to check for the existence of a dynamically attached event listener  Or how can I check the status of the  quot onclick quot      property in the DOM  I have searched the internet just like Stack Overflow for a solution  but no luck  Here is my html   lt a id  quot link1 quot  onclick  quot linkclick event  quot  gt  link 1  lt  a gt   lt a id  quot link2 quot  gt  link 2  lt  a gt   lt  -- without inline onclick handler -- gt   Then in Javascript I attach a dynamically created event listener to the 2nd link  document getElementById  link2   addEventListener  click   linkclick  false    The code runs well  but all my attempts to detect that attached listener fail     test for  link2 - dynamically created eventlistener alert elem onclick      null alert elem hasAttribute  onclick        false alert elem click      function click    native code      btw  what s this   jsFiddle is here  If you click  quot Add onclick for 2 quot  and then  quot  link 2  quot   the event fires well  but the  quot Test link 2 quot  always reports false  Can somebody help

User · Answer

You could in theory piggyback addEventListener and removeEventListener to add remove a flag to the  this  object  Ugly and I haven t tested

User · Answer

Possible duplicate  Check if an element has event listener on it  No jQuery Please find my answer there   Basically here is the trick for Chromium   Chrome  browser    getEventListeners document querySelector  your-element-selector

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Here s a script I used to check for the existence of a dynamically attached event listener  I used jQuery to attach an event handler to an element  then trigger that event  in this case the  click  event   This way I can retrieve and capture event properties that will only exist if the event handler is attached   var eventHandlerType       contentDiv   on  click   clickEventHandler  triggerHandler  click     function clickEventHandler e        eventHandlerType   e type     if  eventHandlerType      click         console log  EventHandler  click  has been applied

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tl dr  No  you cannot do this in any natively supported way     The only way I know to achieve this would be to create a custom storage object where you keep a record of the listeners added  Something along the following lines      Create a storage object     var CustomEventStorage         Step 1  First  you will need a function that can traverse the storage object and return the record of an element given the element  or false       The function that finds a record in the storage by a given element     function findRecordByElement  element           Iterate over every entry in the storage object         for  var index   0  length   CustomEventStorage length  index  lt  length  index                 Cache the record             var record   CustomEventStorage index               Check whether the given element exists             if  element    record element                   Return the record                 return record                          Return false by default         return false      Step 2  Then  you will need a function that can add an event listener but also insert the listener to the storage object      The function that adds an event listener  while storing it in the storage object     function insertListener  element  event  listener  options           Use the element given to retrieve the record         var record   findRecordByElement element           Check whether any record was found         if  record               Normalise the event of the listeners object  in case it doesn t exist             record listeners event    record listeners event                   else              Create an object to insert into the storage object             record                 element  element              listeners                            Create an array for event in the record             record listeners event                    Insert the record in the storage             CustomEventStorage push record                 Insert the listener to the event array         record listeners event  push listener           Add the event listener to the element         element addEventListener event  listener  options       Step 3  As regards the actual requirement of your question  you will need the following function to check whether an element has been added an event listener for a specified event      The function that checks whether an event listener is set for a given event     function listenerExists  element  event  listener           Use the element given to retrieve the record         var record   findRecordByElement element           Check whether a record was found  amp  if an event array exists for the given event         if  record  amp  amp  event in record listeners               Return whether the given listener exists             return    record listeners event  indexOf listener                 Return false by default         return false      Step 4  Finally  you will need a function that can delete a listener from the storage object      The function that removes a listener from a given element  amp  its storage record     function removeListener  element  event  listener  options           Use the element given to retrieve the record         var record   findRecordByElement element           Check whether any record was found and  if found  whether the event exists         if  record  amp  amp  event in record listeners               Cache the index of the listener inside the event array             var index   record listeners event  indexOf listener               Check whether listener is not -1             if   index                   Delete the listener from the event array                 record listeners event  splice index  1                         Check whether the event array is empty or not             if   record listeners event  length                   Delete the event array                 delete record listeners event                           Add the event listener to the element         element removeEventListener event  listener  options         Snippet    x000D   x000D  window onload   function      x000D    var x000D         Cache the test element     x000D      element   document getElementById  test    x000D   x000D         Create an event listener     x000D      listener   function  e    x000D        console log e type    triggered     x000D         x000D   x000D       Insert the listener to the element     x000D    insertListener element   mouseover   listener   x000D   x000D       Log whether the listener exists     x000D    console log listenerExists element   mouseover   listener    x000D   x000D       Remove the listener from the element     x000D    removeListener element   mouseover   listener   x000D   x000D       Log whether the listener exists     x000D    console log listenerExists element   mouseover   listener    x000D     x000D   lt  -- Include the Custom Event Storage file -- gt  x000D   lt script src    https   cdn rawgit com angelpolitis custom-event-storage master main js  gt  lt  script gt  x000D   x000D   lt  -- A Test HTML element -- gt  x000D   lt div id    test  style    background  000  height 50px  width  50px  gt  lt  div gt  x000D   x000D   x000D      Although more than 5 years have passed since the OP posted the question  I believe people who stumble upon it in the future will benefit from this answer  so feel free to make suggestions or improvements to it

User · Answer

There is no way to check whether dynamically attached event listeners exist or not     The only way you can see if an event listener is attached is by attaching event listeners like this   elem onclick   function      console log  1      You can then test if an event listener was attached to onclick by returning   elem onclick  or something similar

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What I would do is create a Boolean outside your function that starts out as FALSE and gets set to TRUE when you attach the event  This would serve as some sort of flag for you before you attach the event again  Here s an example of the idea      initial load var attached   false      this will only execute code once doSomething   function      if   attached       attached   true      code          attach your function with change event window onload   function      var txtbox   document getElementById  textboxID      if  window addEventListener       txtbox addEventListener  change   doSomething  false       else if window attachEvent       txtbox attachEvent  onchange   doSomething

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If I understand well you can only check if a listener has been checked but not which listener is presenter specifically    So some ad hoc code would fill the gap to handle your coding flow  A practical method would be to create a state using variables  For example  attach a listener s checker as following    var listenerPresent false   then if you set a listener just change the value   listenerPresent true   then inside your eventListener  s callback you can assign specific functionalities inside and in this same way  distribute the access to functionalities depending of some state as variable for example   accessFirstFunctionality false accessSecondFunctionality true accessThirdFunctionality true

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I just found this out by trying to see if my event was attached       if you do    item onclick   it will return  null   but if you do    item hasOwnProperty  onclick     then it is  TRUE   so I think that when you use  addEventListener  to add event handlers  the only way to access it is through  hasOwnProperty   I wish I knew why or how but alas  after researching  I haven t found an explanation

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It seems odd that this method doesn t exist  Is it time to add it finally    If you wanted to you could something like the following    var  addEventListener   EventTarget prototype addEventListener  var  removeEventListener   EventTarget prototype removeEventListener  EventTarget prototype events       EventTarget prototype addEventListener   function name  listener  etc      var events   EventTarget prototype events    if  events name     null        events name               if  events name  indexOf listener     -1        events name  push listener           addEventListener name  listener      EventTarget prototype removeEventListener   function name  listener      var events   EventTarget prototype events     if  events name     null  amp  amp  events name  indexOf listener     -1        events name  splice events name  indexOf listener   1           removeEventListener name  listener      EventTarget prototype hasEventListener   function name      var events   EventTarget prototype events    if  events name     null        return false         return events name  length

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I did something like that  const element   document getElementById  div     if  element getAttribute  listener        true          element addEventListener  click   function  e             const elementClicked   e target           elementClicked setAttribute  listener    true             console log  event has been attached               Creating a special attribute for an element when the listener is attached and then checking if it exists

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Just remove the event before adding it    document getElementById  link2   removeEventListener  click   linkclick  false   document getElementById  link2   addEventListener  click   linkclick  false

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You could always check manually if your EventListener exist using Chrome inspector for example  In Element tab you have the traditional  Styles  subtab and close to it another one    Event Listeners   Which will give you the list of all EventListeners with their linked elements

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I usually attach a class to the element then check if the class exist like this  let element   document getElementsById  quot someElement quot     if  element classList contains  attached-listener       element addEventListener  quot click quot   this itemClicked    element classList add  attached-listener

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I just wrote a script that lets you achieve this  It gives you two global functions  hasEvent Node elm  String event  and getEvents Node elm  which you can utilize  Be aware that it modifies the EventTarget prototype method add RemoveEventListener  and does not work for events added through HTML markup or javascript syntax of elm on event        More info at GitHub  Live Demo  Script   var hasEvent getEvents  function   function b a b c  c a dataset events      b a dataset events a dataset events replace new RegExp b      function c a c  var d EventTarget prototype a  EventListener   return function a e f g h  this dataset events   this dataset events     var i hasEvent this a  return c amp  amp i   c amp  amp  i  h amp  amp h    1   d call this a e f  b this a c  g amp  amp g    0   hasEvent function a b  var c a dataset events return c new RegExp b  test c   1  getEvents function a  return a dataset events replace              g     split      filter function a  return     a    EventTarget prototype addEventListener c  add   0  EventTarget prototype removeEventListener c  remove   1

User · Answer

There doesn t appear to be a cross browser function that searches for events registered under a given element   However  it is possible to view the call back functions for elements in some browsers using their development tools  This can be useful when attempting to determine how a web page functions or for debugging code   Firefox  First  view the element in the Inspector tab within the developer tools  This can be done    On the page by right clicking on the item on the web page that you want to inspect and selecting  Inspect Element  from the menu  Within the console by using a function to select the element  such as document querySelector  and then clicking the icon beside the element to view it in the Inspector tab    If any events were registered to the element  you will see a button containing the word Event beside the element  Clicking it will allow you to see the events that have been registered with the element  Clicking the arrow beside an event allows you to view the callback function for it   Chrome  First  view the element in the Elements tab within the developer tools  This can be done    On the page by right clicking on the item on the web page that you want to inspect and selecting  Inspect  from the menu Within the console by using a function to select the element  such as document querySelector  right clicking the the element  and selecting  Reveal in Elements panel  to view it in the Inspector tab    Near the section of the window that shows the tree containing the web page elements  there should be another section with a tab entitled  Event Listeners   Select it to see events that were registered to the element  To see the code for a given event  click the link to the right of it   In Chrome  events for an element can also be found using the getEventListeners function  However  based on my tests  the getEventListeners function doesn t list events when multiple elements are passed to it  If you want to find all of the elements on the page that have listeners and view the callback functions for those listeners  you can use the following code in the console to do this   var elems   document querySelectorAll        for  var i 0  i  lt   elems length  i          var listeners   getEventListeners elems i         if  Object keys listeners  length  lt  1            continue             console log elems i         for  var j in listeners            console log  Event    j            for  var k 0  k  lt  listeners j  length  k                  console log listeners j  k  listener                         Please edit this answer if you know of ways to do this in the given browsers or in other browsers

[javascript] How to check whether dynamically attached event listener exists or not?

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