JavaScript check if browser extension is installed for Chrome, Firefox and Opera

Question

I want to show custom bar ( notification like: Install our plugin. ) on our site if extension is not installed for Chrome, Firefox and Opera. None will be displayed if extension is installed.

The idea is:
1. User visits our site and a bar with message appears - "Hey, install our extensions".
2. User clicks on install and extension is installed. No need for the user to open it.
3. User visit our site again and the bar does not appear again as the extension is installed.
We talk only for installation check, not cookies.

So far I was able to find solutions only for Chrome: http://blog.kotowicz.net/2012/02/intro-to-chrome-addons-hacking.html
and Firefox: http://webdevwonders.com/detecting-firefox-add-ons/
Opera is never mentioned.

Is there a way to do it for Opera, based on the idea of checking for a local resource which is part of the extension:
chrome-extension://--Extension ID--/blank.gif
Firefox chrome://firebug/content/blank.gif

Or else: Is there a simple way to check if extension is installed for Chrome, Firefox and Opera?

Answer 1

If the extension is willing to cooperate, it could advertise its presence to the document easily. For example:

---

The extension could do

window.$$myExt = ...

Then you can detect the extension by

if(typeOf $$myExt !== 'undefined'){...

(or any variation thereof)

Obtaining the page window is somewhat tricky at least

---

The extension could do

document.body.classList.add("myExt-visited")

Then you could detect the extension by

if(document.body.classList.contains("myExt-visited")){...

---

The extension could do

document.body.innerHTML += "<div id='myExt-toolbar'>..."
// or $('body').append("<div id='myExt-toolbar'>...");

then you could detect the extension by

if(document.getElementByID("myExt-toolbar")){...
// or if($("#myExt-toolbar").length){...

---

alternatively, you could do

<div id="myExt-replacement">
   ...

and the extension would do

var replacement = document.getElementByID("myExt-replacement");
replacement && replacement.remove();

---

or you could do

function onMyExtExists(){
  ...
}

and the extension would do

onMyExtExists && onMyExtExists();