2.1.57 [ECMA-262-1999] Section 15.2.4.6, Object.prototype.isPrototypeOf (V)

V0091:

When the isPrototypeOf method is called with argument V, the following steps are taken:

  1. Let O be #this object# __the result of calling ToObject passing the this value as the argument__.

  2. If V is not an object, return false.

    1. __If O and V refer to the same object, return true.__

3. Let V be the value of the [[Prototype]] property of V.

4. if V is null, return false.

5. If O and V refer to the same object or if they refer to objects joined to each other ([ECMA-262-1999] section 13.1.2), return true.

6. Go to step 3.

In JScript 5.x, the isPrototypeOf method returns true rather than false if the this value and the argument are the same object.