x instanceof y where y is a class name, returns a boolean value.
delete o.x removes property x from object o, true if succeeds or property did not exist. Does not set the property to undefined. If x refers to an object, that is not deleted.
void discards a result and returns the value undefined. Eg,
<a href="javascript:void window.open();">Open</a>
which performs the function without trying to link. NB: This shows another way of invoking JavaScript.