Generic JavaScript Page onLoad Event Handler

//setup onload function
if(typeof window.addEventListener != 'undefined')
{
    //.. gecko, safari, konqueror and standard
    window.addEventListener('load', generic, false);
}
else if(typeof document.addEventListener != 'undefined')
{
    //.. opera 
    document.addEventListener('load', generic, false);
}
else if(typeof window.attachEvent != 'undefined')
{
    //.. win/ie
    window.attachEvent('onload', generic);
}

//** remove this condition to degrade older browsers
else
{
    //.. mac/ie5 and anything else that gets this far
    
    //if there's an existing onload function
    if(typeof window.onload == 'function')
    {
        //store it
        var existing = onload;
        
        //add new onload handler
        window.onload = function()
        {
            //call existing onload function
            existing();
            
            //call generic onload function
            generic();
        };
    }
    else
    {
        //setup onload function
        window.onload = generic;
    }
}

As referenced from Brothercake.com