What's the difference between isPrototypeOf and instanceof in Javascript?

In some of my own older code, I use the following:

Object.prototype.instanceOf = function( iface )
{
 return iface.prototype.isPrototypeOf( this );
};

Then I do (for example)

[].instanceOf( Array )

This works, but it seems the following would do the same:

[] instanceof Array

Now, surely this is only a very simple example. My question therefore is:

Is a instanceof b ALWAYS the same as b.prototype.isPrototypeOf(a) ?