How to access object prototype in javascript?

var f = function();
var instance = new f();

If you know name of instance class function, you can simply access prototype as:

var prototype = f.prototype;
prototype.someMember = someValue;

If you don't:

1)

var prototype = Object.getPrototypeOf(instance);
prototype.someMember = someValue;

2) or

var prototype = instance.__proto__;
prototype.someMember = someValue;

3) or

var prototype = instance.constructor.prototype; // works only if constructor is properly assigned and not modified
prototype.someMember = someValue;

For compatibility you can place into your code the following snippet (and use always Object.getPrototypeOf(instance) to return prototype):

if(!Object.getPrototypeOf) {

  if(({}).__proto__ === Object.prototype && ([]).__proto__ === Array.prototype) {

    Object.getPrototypeOf = function getPrototypeOf(object) {
      return object.__proto__;
    };

  } else {

    Object.getPrototypeOf = function getPrototypeOf(object) {

      // May break if the constructor has been changed or removed
      return object.constructor ? object.constructor.prototype : void 0;

    };

  }
}

UPDATE:

According to ECMA-262 6th Edition (June 2015) __proto__ property is standardized as additional feature for Web browsers. All latest editions of top browsers supports it now. Read more about __proto__:

• MDN: Object.prototype.__proto__

• EDMA-262 6th Edition (June 2015): B.2.2.1 Object.prototype.__proto__

It looks like

Object.getPrototypeOf(passedObject);

will work for this, and is compatible with modern browsers.

Here are the compatibility tables on MDN

var F = function(){};
var object1 = new F();
alert(object1.constructor === F);
alert(object1.constructor.prototype === F.prototype);