Polyfill for Object.getPrototypeOf -- It will work unless your constructor's prototype doesn't have a constructor property (which means you assigned another object as prototype and didn't give it a constructor property...)

Object.getPrototypeOf.js

if ( typeof Object.getPrototypeOf !== "function" ) {
	( function ( ) {
		function getPrototypeValue( o, p ) {
			if ( o.hasOwnProperty( p ) ) {
				var ownValue = o[ p ];
				if ( delete o[ p ] ) {
					var prototypeValue = o[ p ];
					o[ p ] = ownValue;
					return prototypeValue;
				} else {
					return o[ p ];
				}
			} else {
				return o[ p ];
			}
		}
		if ( typeof "".__proto__ === "object" ) {
			Object.getPrototypeOf = function( object ) {
				return getPrototypeValue( object, '__proto__' );
			};
		} else {
			Object.getPrototypeOf = function( object ) {
				getPrototypeValue( object, 'constructor' ).prototype;
			};
		}
	}( ) );
}

Sorry this doesn't seem to work in IE8 or less. Here is a code sample that demonstrates the problem:

if ( typeof Object.getPrototypeOf2 !== "function" ) {
( function ( ) {
function getPrototypeValue( o, p ) {
if ( o.hasOwnProperty( p ) ) {
var ownValue = o[ p ];
if ( delete o[ p ] ) {
var prototypeValue = o[ p ];
o[ p ] = ownValue;
return prototypeValue;
} else {
return o[ p ];
}
} else {
return o[ p ];
}
}
if ( typeof "".proto === "object" ) {
Object.getPrototypeOf2 = function( object ) {
return getPrototypeValue( object, 'proto' );
};
} else {
Object.getPrototypeOf2 = function( object ) {
getPrototypeValue( object, 'constructor' ).prototype;
};
}
}( ) );
}

function Parent() {}
var parent = new Parent();
function Child() {}
Child.prototype = parent;
var child = new Child();

// child inherits directly from parent. this makes sense.
console.log(Object.getPrototypeOf2(child) === parent); // false in ie 8 or less

That's normal...
There is no "normal" way you can get the prototype of an object in ES 3.
There is proto but it isn't standard.
And there is getting obj.constructor.prototype.
In your case, you redefine the prototype and therefore, it doesn't have a constructor property anymore so it fails. And there is nothing I can do about it.

function C(){}
C.prototype = {};
Object.getPrototypeOf( new C( ) ); // FAIL

function C(){}
C.prototype = {constructor: C};
Object.getPrototypeOf( new C( ) ); // OK

Normal or not, it shows that your getPrototypeOf does not behave the same as the getPrototypeOf implemented in newer browsers. A polyfill needs to have the exact same behavior.

I just used this to reference it in an email I sent to John Resig about his blog post: http://ejohn.org/blog/objectgetprototypeof/
I really don't care whether it does work or not since I never used getPrototypeOf and probably never will.

Updated the name of this gist. Happy?

And how the hell did you find that Gist? -.-

This was a top Google result for "getProtoTypeOf polyfill". I think your updating the description will really help people who stumble across it, so thanks!

This is incorrect:

// child inherits directly from parent. this makes sense.
console.log(Object.getPrototypeOf2(child) === parent); // false in ie 8 or less

Child is the prototype of child, with Parent being the prototype of Child

It needs the constructor property to work properly.

What makes this work consistently is to enforce it with an Object.create polyfill that adds proto to all newly created objects when called, that way your else never gets used, but I suppose I am digressing...

When you say proto you mean proto ? Because there are some browser supproting proto and not Object.create. So it would create some weird bugs.
And if it is proto then you need to add an else if.

if you polyfill Object.create to add the proto then you won't hit the else above.

You'd still need it to work on objects created with the new operator.