Event Utility

Here is an object for adding and removing event handlers.

var EventUtility = {
	addHandler: function(element, type, handler){
		if (element.addEventListener){
			element.addEventListener(type, handler, false);
		} else if (element.attachEvent){
			element.attachEvent("on" + type, handler);
		} else {
			element["on" + type] = handler;
		}
	},
	getClipboardText: function(event){
		var clipboardData = (event.clipboardData || window.clipboardData);
		return clipboardData.getData("text");
	},
	setClipboardText: function(event, value){
		if (event.clipboardData){
			return event.clipboardData.setData("text/plain", value);
		} else if (window.clipboardData){
			return window.clipboardData.setData("text", value);
		}
	},
	getCharCode: function(event){
		if (typeof event.charCode == "number"){
			return event.charCode;
		} else {
			return event.keyCode;
		}
	},
	getEvent: function (event) {
		return event ? event : window.event;
	},
	getTarget: function (event) {
		return event.target || event.srcElement;
	},
	preventDefault: function (event) {
		if (event.preventDefault) {
			event.preventDefault();
		} else {
			event.returnValue = false;
		}
	},
	removeHandler: function(element, type, handler){
		if (element.removeEventListener){
			element.removeEventListener(type, handler, false);
		} else if (element.detachEvent){
			element.detachEvent("on" + type, handler);
		} else {
			element["on" + type] = null;
		}
	},
	stopPropagation: function (event) {
		if (event.stopPropagation) {
			event.stopPropagation();
		} else {
			event.cancelBubble = true;
		}
	}
};

Using the EventUtility object:

EventUtility.addHandler(document.getElementById("textbox"), "keypress", function(event){
	event = EventUtility.getEvent(event);
	var target = EventUtility.getTarget(event);
	var charCode = EventUtility.getCharCode(event);
	if (!/[a-zA-Z]/.test(String.fromCharCode(charCode))){
		EventUtility.preventDefault(event);
	}
});

One thing worth noting here is that users may paste anything into this field so, on the blur event, code should double check that there are only letters entered.