My book (Flanagan's JavaScript: The Definitive Guide, 5th ed.)
implies on page 111 that the following two constructs are equivalent:

  ( x.constructor == Foo )

and

  ( x instanceof Foo )

The author states that the instanceof operator computes its result
by examining the value of its first argument's the constructor
property.

However, I've recently ran into a situation that contradicts this.

I've been trying to understand jQuery better, with the aid of
Firefox's Firebug debugger.  At one breakpoint, there's one variable
(I'll call it x) for which

  x instanceof jQuery

is true, but

  x.constructor == jQuery

is false.

In fact, x.constructor is Object, but (Object instanceof jQuery)
is false.

Could anyone explain to me what's going on?

TIA!

Kynn