Douglas Crockford "Javascript: Good Details" Chapter 5.5

This is very similar to this question.

My question is more about sample code.

the code:

var eventuality = function (that) {
    var registry = {};
    that.fire = function (event) {
// Fire an event on an object. The event can be either
// a string containing the name of the event or an
// object containing a type property containing the
// name of the event. Handlers registered by the 'on'
// method that match the event name will be invoked.
        var array,
            func,
            handler,
            i,
            type = typeof event === 'string' ?
                    event : event.type;
// If an array of handlers exist for this event, then
// loop through it and execute the handlers in order.
        if (registry.hasOwnProperty(type)) {
            array = registry[type];
            for (i = 0; i < array.length; i += 1) {
                handler = array[i];
// A handler record contains a method and an optional
// array of parameters. If the method is a name, look
// up the function.
                func = handler.method;
                if (typeof func === 'string') {
                    func = this[func];
                }
// Invoke a handler. If the record contained
// parameters, then pass them. Otherwise, pass the
// event object.
                func.apply(this,
                    handler.parameters || [event]);
            }
        }
        return this;
    };
    that.on = function (type, method, parameters) {
// Register an event. Make a handler record. Put it
// in a handler array, making one if it doesn't yet
// exist for this type.
        var handler = {
            method: method,
            parameters: parameters
        };
        if (registry.hasOwnProperty(type)) {
            registry[type].push(handler);
        } else {
            registry[type] = [handler];
        }
        return this;
    };
    return that;
}

in the code, I do not understand this line func = handler.method;.

How it works? I mean handler.method should be undefined, right?

thank