function foo (event) {
    // doesn't have a return statement
    // so this function returns undefined
    if (event) {
        alert('foo() was invoked by an event listener.');
    } else {
        alert('foo() was invoked some other way.\n(e.g., while adding the event listener, to get its return value)');
    }
}

// this listener will run the foo function, get its return value, *then* attempt to add the listener
window.addEventListener('click', foo(), false);

// since foo() doesn't return anything, it actually does this
window.addEventListener('click', undefined, false);
// foo() does get executed once, because you invoked it to get its return value
// but it will not actually add a listener

// now let's add a real listener, and our foo() function will tell us some info whenever we call it
window.addEventListener('click', foo, false);