Question

Hello people of Stackoverflow! I have been going through the Mozilla Developer Network’s JavaScript Guide and came across this function on the Details of the object model page:

The function is to check if an object is an instance of an object constructor:

function instanceOf(object, constructor) {
   while (object != null) {
      if (object == constructor.prototype)
         return true;
      if (typeof object == 'xml') {
        return constructor.prototype == XML.prototype;
      }
      object = object.__proto__;
   }
   return false;
}

My question is that, from the same page it says that is chris is an object of type Engineer then the following code returns true:

chris.__proto__ == Engineer.prototype;

However, in the above instanceOf function, it uses the following comparison expression to check if an object is an instance of a constructor function:

object == constructor.prototype

Should the expression not be:

object.__proto__ == constructor.prototype

Or am I missing a point here? Thank you all for your help and time in advance!

Answer 1

You’re missing the statement object = object.__proto__; at the bottom of the while loop… This traverses the prototype-chain. The object variable contains the current object from that chain for each step of the traversal.