Determining a JS object's prototype

Question

What is the best way to determine a JavaScript object's prototype? I am aware of the following two methods, but I'm not sure which is the "best" way (or if there's a better preferred way) in terms of cross-browser support.

if (obj.__proto__ === MY_NAMESPACE.Util.SomeObject.prototype) {
    // ...
}

or

if (obj instanceof MY_NAMESPACE.Util.SomeObject) {
    // ...
}

I would say instanceof because the other method seems a bit hackish, and could easily stop working on the next edition of ECMAScript. — Alex W
– Alex W, Commented Jan 7, 2013 at 18:18
There is a good article that helps you a lot: http://ejohn.org/blog/objectgetprototypeof/ — Ramin Omrani
– Ramin Omrani, Commented Jan 7, 2013 at 18:23

apsillers · Accepted Answer · 2013-01-07 18:26:12Z

6

instanceof is prefered. __proto__ is nonstandard -- specifically, it doesn't work in Internet Explorer.

Object.getPrototypeOf(obj) is an ECMAScript 5 function that does the same thing as __proto__.

Note that instanceof searches the whole prototype chain, whereas getPrototypeOf only looks one step up.

Some usage notes:

new String() instanceof String  // true

(new String()).__proto__ == String // false!
                                   // the prototype of String is (new String(""))
Object.getPrototypeOf(new String()) == String // FALSE, same as above

(new String()).__proto__ == String.prototype            // true! (if supported)
Object.getPrototypeOf(new String()) == String.prototype // true! (if supported)

edited Jan 7, 2013 at 18:26

answered Jan 7, 2013 at 18:18

apsillers

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Comments

Sean Vieira · Accepted Answer · 2013-01-07 18:25:59Z

3

instanceof is standard, while __proto__ is not (yet - it will most likely be standard in ECMAScript 6).

edited Jan 7, 2013 at 18:25

answered Jan 7, 2013 at 18:19

Sean Vieira

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