How can I see a Javascript object's prototype chain?

Question

Given the following code:

function a() {}
function b() {}
b.prototype = new a();
var b1 = new b();

We can stay that a has been add

Answer 1

edit — This answer is from 2010, and quite obsolete. Since then the language has added the Object.getPrototypeOf() API, which vastly simplifies the process.

---

You could use the "constructor" property of the object to find the prototype there, and then chain along that until you reached the end of the rainbow.

function getPrototypes(o) {
  return (function gp(o, protos) {
    var c = o.constructor;
    if (c.prototype) {
      protos.push(c.prototype);
      return gp(c.prototype, protos);
    }
    return protos;
  })(o, []);
}

(maybe) (or maybe not :-) give me a sec) (well crap; I think it's possible but ignore that code)

[edit] wow this is totally blowing my mind - that function's close but not quite right; setting up a chain of prototypes is weird and I'm feeling scared and lonely. I suggest paying attention only to the awesome @CMS above.

Answer 2

Well, the prototype link between objects ([[Prototype]]) is internal, some implementations, like the Mozilla, expose it as obj.__proto__.

The Object.getPrototypeOf method of the ECMAScript 5th Edition is what you're needing, but it isn't implemented right now on most JavaScript engines.

Give a look to this implementation by John Resig, it has a pitfall, it relies on the constructor property of engines that don't support __proto__:

if ( typeof Object.getPrototypeOf !== "function" ) {
  if ( typeof "test".__proto__ === "object" ) {
    Object.getPrototypeOf = function(object){
      return object.__proto__;
    };
  } else {
    Object.getPrototypeOf = function(object){
      // May break if the constructor has been tampered with
      return object.constructor.prototype;
    };
  }
}

Remember that this is not 100% reliable, since the constructor property is mutable on any object.