Optimize jquery plugin "on" method extension
I dont know if there is a better way to do it, but it seems quite smart and minimally intrusive.

From a once over style perspective:


function (key, element) &lt;- since you dont use element, you can remove it from the signature
This

$.each(fctsToObserve, function (key, element) {
  fctsObserveKeys += "hasChanged." + key + " ";
});


would have been better served with Array.reduce, not sure if you want to support browsers that don't have Array.reduce
Naming.. Any code reviewer would be remiss if they did not point that you abbreviate your variables too much which makes the code needlessly harder to understand
In the if in $.fn.on = function () { you should use curly braces or a least a newline.
There is no point in declaring var namespace here:

observeMethods: function (namespace) {
    var namespace = namespace ? "." + namespace : "";


just go for

observeMethods: function (namespace) {
    namespace = namespace ? "." + namespace : "";

Comments, some of the more trickier concepts could have used a line of comment