How do I add and remove an event listener using a function with parameters?

Question

Sorry if this is a common question, but I couldn't find any answers that seemed pertinent through searching.

If I attach an event listener like this:

window.addEventListener('scroll', function() { check_pos(box); }, false);

it doesn't seem to work to try to remove it later, like this:

window.removeEventListener('scroll', function() { check_pos(box); }, false);

I assume this is because the addEventListener and removeEventListener methods want a reference to the same function, while I've provided them with anonymous functions, which, while identical in code, are not literally the same.

How can I change my code to get the call to removeEventListener to work? The "box" argument refers to the name of an <iframe> that I'm tracking on the screen; that is, I want to be able to subscribe to the scroll event once for each <iframe> that I have (the quantity varies), and once the check_pos() function measures a certain position, it will call another function and also remove the event listener to free up system resources.

My hunch is that the solution will involve a closure and/or naming the anonymous function, but I'm not sure exactly what that looks like, and would appreciate a concrete example.

Hope that makes sense.

Answer 1

Have you tried maintaining a reference to the anonymous function (like you suggested)?

So:

var listener = function() {
 check_pos(box);
};
window.addEventListener('scroll', listener, false);
...
window.removeEventListener('scroll', listener, false);

Mozilla's docs suggest the same thing.

• Bungle: Thanks, Vivin - correct me if I'm wrong, but I don't think this would allow me to set up multiple event listeners with different values for "box," which is what I need to do. The argument "box" isn't a global variable. For example, just using literal values, I might call window.addEventListener('scroll', function() { do_something('string1'); }, false); and later window.removeEventListener('scroll', function() { do_something('string1'); }, false); and later window.addEventListener('scroll', function() { do_something('string2'); }, false);, etc.
• Vivin Paliath: There is a way to add multiple event listeners by using closures (See stackoverflow.com/questions/2276961/…). However I'd suggest using jQuery if you want to do some serious event-handling. It makes it much easier.
• Ambuj Khanna: If remove it by using < window.removeEventListener('scroll', listener, false); > and check again by using < "listener" in window" > so it is still giving me back "true". Any help?

Answer 2

var listener;
listener = function(){
if( window.target != anotherEvent.target )
{
 ...CODE where
 window.removeEventListener('click', listener , false);
};
window.addEventListener('click', listener ,false);

Answer 3

document.getElementById("yourId").removeEventListener("click",yourfunction1);
document.getElementById("yourId").addEventListener("click",yourfunction2);
function yourfunction1(){
//write code here
alert(1);
}
function yourfunction2(){
//write code here
alert(2);
}

<button type="button" onclick="yourfunction1()" id="yourId">Button</button>