Javascript Avoid Overwriting Listener

Question

I'm developing a piece of javascript which will run inside a non-friendly iframe. Fortunally I have an interface which permits me to run javascript inside the iframe parent page but this is not the problem.

The problem is: in my code I need to attach a function to the window.onscroll event of the iframe parent page. Unfortunately I don't know from the beginning on which websites my code will run so I don't know if they already have a function attached to window.onscroll

What will happen if I do a window.onscroll = function(){...} while the website already have one? Will it be overwritten? If yes: is there a way to avoid this?

I hope I've explained myself. Thank you in advance!

It depends on how events are bound. Check out http://stackoverflow.com/questions/6348494/addeventlistener-vs-onclick — j08691, May 16 '14 at 14:29

score 1 · Accepted Answer · answered May 16 '14 at 14:21

1

You can do:

var oldHandler = null;

if('function' == typeof window.onscroll) {
  oldHandler = window.onscroll; 
}

window.onscroll = function(ev) {
  // your stuff...
  if(oldHandler) oldHandler.call(window, ev);
}

answered May 16 '14 at 14:21

marekful

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Also note that using jQuery, handlers are automatically "merged". So `$(window).scroll(function() {});` will preserve the original handler. – marekful May 16 '14 at 15:05
Thank you but I'm not using jQuery ;) – Andrea Silvestri May 16 '14 at 15:07
Sorry for letting you wait! But your answer is good for me =D Thank you! – Andrea Silvestri May 30 '14 at 10:24

Javascript Avoid Overwriting Listener

1 Answers1