Question

I’m looking for override event and still preform it’s actions.

Meaning something like this:

var oldOnError = window.onerror;
window.onerror = function(msg, url, line) {
 oldOnError();
};

Now this will get executed however, onerror event could accept several arguments and I don’t and can’t know which are there.

So :

• Is there a simple solution for this?
• If not, how can i know what parameters oldOnError expecting to get?

Answer 1

Why not use addEventListener()

var listener = function(msg, url, line) {
 // your code
};

if (window.addEventListener) {
    window.addEventListener('error', listener, false);
} else if ( window.attachEvent ) { // for versions previous to IE9
    window.attachEvent( 'onerror', listener );
}

That way you do not override existing event listeners.

The other thing to note is that even though you specify 3 arguments your function is passed an event object when called. So really your function should be defined as follows.

var listener = function( event ) {
    // your code
}

Have a look at the MDN site here for more information