Forcing a typecast of a custom object in javascript

Question

Suppose I have 2 constructors and a member function defined

function A() {}
function B() {}
A.prototype.a = function(){}

And I've instantiated a B. How can I force a conversion to make it an A?

o = new B();
//What should I put here?
o.a();

I'm new to js. And I have a feeling that whether o mentioned above is an A or a B is merely controlled by a reference to a prototype. Therefore, I feel typecast should be possible.

Answers, as well as explanations that can help me understand js objects, are welcomed.

T.J. Crowder · Accepted Answer · 2012-12-02 09:32:49Z

3

You can't change (that is, completely swap-out) the prototype of an object once it's been created (yet; in a future version of JavaScript, it's likely to be possible).

Objects in JavaScript are enormously malleable, though. You can do this, for instance:

o.a = A.prototype.a;

...and then o will have the a function. If you wanted to give it all of the enumerable properties of A.prototype (including any from its prototype):

var name;
for (name in A.prototype) {
    o[name] = A.prototype[name];
}

The prototype property of functions is a completely normal, boring object assigned to a normal, boring property on the function. When you use new to create an object using that function (which is called a "constructor function"), the function's prototype property is assigned to the newly-created object as its underlying prototype (which has no name you can access in code [yet]).

As of ES5 (reasonably supported in Chrome, Firefox, Opera, and IE9; not in IE8 and earlier), you can get the prototype of an object via Object.getPrototypeOf, and you can create objects assigning them a prototype without going through constructor functions by using Object.create, but you can't change what an object's prototype is once it's been created.

Some JavaScript engines (like the one in Firefox) have a currently-non-standard extension called __proto__ which lets you directly access and manipulate the prototype of an object, by treating it as a property. E.g.:

o.__proto__ = A.prototype;

...but that is not standard at present. It may well be part of the next standard, though. If you're interested in information about the upcoming ES6, you can find draft specifications and such here.

edited Dec 2, 2012 at 9:32

answered Dec 2, 2012 at 9:18

T.J. Crowder

1.1m201 gold badges2k silver badges2k bronze badges

Sign up to request clarification or add additional context in comments.

14 Comments

John Dvorak Over a year ago

Why is __proto__ not mentioned?

John Dvorak Over a year ago

Note that setPrototypeOf is already in the drafts and confirmed: bugs.ecmascript.org/show_bug.cgi?id=264

T.J. Crowder Over a year ago

@JanDvorak: I was just adding __proto__.

T.J. Crowder Over a year ago

@JanDvorak: I think we can cut them slack, A) because they did so much in IE9 to bring it forward, and B) because there's been a lot of back-and-forth on __proto__. Many (including me) feel that it's not at all smart to have a property-like-thing with a weird name for this, setPrototypeOf would have been entirely sufficient. But apparently we're losing this battle.

Haozhun Over a year ago

Very informative answer. Also, the discussion between you and @JanDvorak are quite interesting. Thank you.

|

Collectives™ on Stack Overflow

Forcing a typecast of a custom object in javascript

1 Answer 1

14 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

14 Comments

Your Answer

Sign up or log in

Post as a guest

Related