事件委托

document.onclick = function(event){        //IE doesn't pass in the event object        event = event || window.event;          //IE uses srcElement as the target        var target = event.target || event.srcElement;          switch(target.id){                case "help-btn":                        openHelp();                        break;                case "save-btn":                        saveDocument();                        break;                case "undo-btn":                        undoChanges();                        break;                //others?        }};