What does 'is an instance of' mean in Javascript?

Question

The answer to this question: What is the initial value of a JavaScript function's prototype property?

has this sentence:

The initial value

Answer 1

You're right that JavaScript doesn't have classes (yet), but it does have constructor functions, an instanceof operator that defines a relationship between objects and constructors, and a form of inheritance based on prototype chains.

obj instanceof ctor is true when ctor.prototype is on obj's prototype chain.

Modulo the caveat below, you could implement instanceof in EcmaScript 5 thus

function isInstanceOf(obj, ctor) {
  var proto = ctor.prototype;
  if (typeof obj === "object" || typeof obj === "function") {
    while (obj) {
      if (obj === proto) { return true; }
      obj = Object.getPrototypeOf(obj);
    }
  }
  return false;
}

Unless you go around reassigning prototypes (o = new MyConstructor(); MyConstructor.prototype = somethingElse) it should be the case that new MyConstructor() instanceof MyConstructor.

Section 15.3.5.3 explains this in detail.

15.3.5.3 [[HasInstance]] (V)

Assume F is a Function object.

When the [[HasInstance]] internal method of F is called with value V, the following steps are taken:

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

2. Let O be the result of calling the [[Get]] internal method of F with property name "prototype".

3. If Type(O) is not Object, throw a TypeError exception.

4. Repeat

   1. Let V be the value of the [[Prototype]] internal property of V.
   2. If V is null, return false.
   3. If O and V refer to the same object, return true.

This isn't the whole story because host objects (like DOM nodes) are allowed to implement the [[HasInstance]] internal method however they like but most browsers implement host objects to behave as closely to native objects as possible.