Question

I currently am working on a bookmarklet that opens an iframe, and sets up a communication of postMessage back and forth. That all works fine.
However, seemingly because the bookmarklet is being loaded as an anonymous function, the listeners are multiplying if I run the bookmarklet more than once on a page.

Is there some sort of way to keep track of these addEventListeners so that they don’t double-up?

Do I need to define the rp_receive_message outside of the anonymous function?

Here’s an example of the code:

var rp_receive_message = function (e) {
    var response = e.data;
    console.log("got message with "+ response);
  };

if (window.addEventListener) {
  window.addEventListener('message', rp_receive_message, false);
} else {
  window.attachEvent('onmessage', rp_receive_message);
}

var s1 = window.document.createElement('iframe');

s1.setAttribute('src', 'http://mydomain.com/iframe.html');
s1.setAttribute('id', 'testiframe');
s1.setAttribute('width', '700');
s1.setAttribute('height', '550');
s1.setAttribute('frameBorder', '0');
s1.setAttribute('onload', 'this.contentWindow.postMessage(window.location.href, "http://mydomain.com/iframe.html");');


document.getElementById('container').appendChild(s1);

Answer 1

Probably this will solve the problem:

window.onmessage = rp_receive_message;

As you suggest, the code below might be enough by itself. I don’t know if addEventListener and attachEvent will add the same function multiple times, but I wouldn’t at all be surprised if they will. I suggest just testing it.

window.rp_receive_message = function(){...}

If you dislike either solution, you’ve got to set up a global variable, which hardly seems any different or greatly superior to above. The global can be a simple boolean to check if the event has been attached, or it can be a list of attached events that you update yourself. AFAIK, and I’m pretty sure, there is no native JS solution to get a list of event listeners have been attached to a particular event. Libraries such as jQuery maintain lists and let you read them; and possibly have other techniques that are elegant solutions to your general problem.