is Object.getPrototypeOf() same as Object.constructor.prototype in Javascript?

Question

Is there a difference between Object.getPrototypeOf(obj) and obj.constructor.prototype? Or are these two referencing the same thing?

Answer 1

NO

It returns the internal [[Prototype]] value.

For example:

var o = Object.create(null);
Object.getPrototypeOf(o); // null
o.constructor.prototype; // error

var p = {};
var o = Object.create(p);
Object.getPrototypeOf(o); // p
o.constructor.prototype; // Object.prototype

o.constructor.prototype only works with objects created through new ConstructorFunction or where you have manually set the Prototype.prototype.constructor === Prototype relationship.

Answer 2

No. In particular, the constructor property of an object is not always set to what you would consider "correct."

An example of where getPrototypeOf works but .constructor.prototype does not:

function F() { }
F.prototype = {
   foo: "bar"
};

var obj = new F();

assert.equal(obj.constructor.prototype, Object.prototype);
assert.equal(Object.getPrototypeOf(obj), F.prototype);

It also fails for typical prototypal inheritance scenarios:

// G prototypally inherits from F
function G() { }
G.prototype = Object.create(F.prototype);
// or: G.prototype = new F();

var obj2 = new G();

assert.equal(obj2.constructor.prototype, Object.prototype);
assert.equal(Object.getPrototypeOf(obj2), G.prototype);
assert.equal(Object.getPrototypeOf(Object.getPrototypeOf(obj2)), F.prototype);