# How to check whether a class derives from another class?

**URL:** <https://discuss.emberjs.com/t/how-to-check-whether-a-class-derives-from-another-class/9323>\
**Category:** Uncategorized\
**Created:** [December 16, 2015, 9:03am UTC](https://discuss.emberjs.com/t/how-to-check-whether-a-class-derives-from-another-class/9323 "2015-12-16T09:03:22Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![ctusch](https://avatars.discourse-cdn.com/v4/letter/c/c77e96/32.png) [@ctusch](https://discuss.emberjs.com/u/ctusch)\
**Post date:** [December 16, 2015, 9:03am UTC](https://discuss.emberjs.com/t/how-to-check-whether-a-class-derives-from-another-class/9323/1 "2015-12-16T09:03:23Z")

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Hey 🙂

I’m trying to detect whether a class which is unknown at runtime derives from a certain base class. The obvious way didn’t work:

```
if (SomeClass.prototype === BaseClass) {
    // do stuff
} 

```

How would I do this in Ember?

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<div class="post-metadata">

**Author:** ![spectras](https://avatars.discourse-cdn.com/v4/letter/s/bbce88/32.png) [@spectras](https://discuss.emberjs.com/u/spectras)\
**Post date:** [December 16, 2015, 1:35pm UTC](https://discuss.emberjs.com/t/how-to-check-whether-a-class-derives-from-another-class/9323/2 "2015-12-16T13:35:25Z")

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Just to be sure we’re talking about the same thing, inheritance does not exist in javascript. Be it ember or not. So there is no such a thing as a base class (or as a class actually).

Rather there are prototype chains, which get linked when you create a new object. You may check whether an object’s prototype chain includes another object using javascript `instanceof` operator.

```
var A = Ember.Object.extend();
var B = A.extend();
var a = A.create();
var b = B.create();

a instanceof A; // true
b instanceof A; // true
B instanceof A; // false
B.prototype instanceof A; // true

```

That is, B itself is not an instance of A, but when you use B to create an object, its prototype will include the link to A. The details of this depend on the object implementation you use. That of Ember keeps a separate object for “classes” and “instances”. `create()` and `extend()` actually share the same purpose of creating a new JS object, they only differ in the way they set it up.

It’s fun by the way, `create()` basically does that: `return new this();`

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<div class="post-metadata">

**Author:** ![ctusch](https://avatars.discourse-cdn.com/v4/letter/c/c77e96/32.png) [@ctusch](https://discuss.emberjs.com/u/ctusch)\
**Post date:** [December 16, 2015, 2:49pm UTC](https://discuss.emberjs.com/t/how-to-check-whether-a-class-derives-from-another-class/9323/3 "2015-12-16T14:49:03Z")

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Oh yeah, of course I have to use instanceof on the prototype! Got that confused. Thanks a lot!