How to trigger an input event with jQuery?

I used the trigger method:

$('#selector').val(quantity).trigger("input");

like this?? Sorry, I'm confused with your writings..

$("#button").click(function(){
    $("#input").trigger("keypress") // you can trigger keypress like this if you need to..
    .val(function(i,val){return val + 'a';});
});

reference: .val(function(index, value));