Appending to an object

How about storing the alerts as records in an array instead of properties of a single object ?

var alerts = [ 
    {num : 1, app:'helloworld',message:'message'},
    {num : 2, app:'helloagain',message:'another message'} 
]

And then to add one, just use push:

alerts.push({num : 3, app:'helloagain_again',message:'yet another message'});