event.toElement.id always undefined when I trigger event click

I am newbie to html and java-script ,trying to fire a click event when page load finished

$(document).ready(function(event){
    $("#london").click(function(event){
       openCity(event,'London');
    });

    $("#london").trigger('click'); //this is automatic click

});

This click is working.But in my function

function openCity(evt, cityName) {
   var id = evt.toElement.id; //id is undefined only when click event triggered
}

id is undefined only when click event triggered,when normal click it contains value.