Question

this page states:

Note: isPrototypeOf differs from
instanceof operator. In the expression
object instanceof AFunction, the
object prototype chain is checked
against AFunction.prototype, not
against AFunction itself

Ok I don’t really get what they are trying to tell us. Isn’t object instanceof AFunction exactly the same as `AFunction.prototype.isPrototypeOf(object)? or am I wrong?

Why do we need the isPrototypeOf at all?

If i ever need to do p.isPrototypeOf(o) couldn’t I just do o instanceof p.constructor ?

Addtionally, is p.isPrototypeOf(o) functionally equivalent to p===Object.getPrototypeOf(o)?

Answer 1

Object constructors are funky things. From this answer:

As Pointy has pointed out, in his
answer

The “constructor” property is a
reference to the function that created
the object’s prototype, not the object
itself.

The usual way to deal with this is to
augment the object’s prototype
constructor property after assigning
to the prototype.

An object’s constructor is not read-only, which is why this is possible to do at all. I could assign any value to p.constructor after p is created, and this would completely break using

o instanceof p.constructor

instead of

p.isPrototypeOf(o)

Further reading

• constructor @ MDC
• What it the significance of the Javascript constructor property?
• OO Programming in JS
• Constructors considered mildly confusing

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Edit re: OP edit

Addtionally, is p.isPrototypeOf(o) functionally equivalent to p===Object.getPrototypeOf(o)?

Those are more similar than your original question, aside from the fact that Object.getPrototypeOf wasn’t introduced until JavaScript 1.8.1? See John Resig – Object.getPrototypeOf. Perhaps more relevant, the two functions are different in the spec! _{^{(warning, PDF link)}}