CoffeeScript: Using instanceof vs Class.constructor.name

Question

If I have a class:

class Haha constructor: (@lolAmount = 1) -> alert @lolAmount

And I want to check if an object is of the right class, Is it always safe to use constructor.name:

haha = new Haha() unless haha.constructor.name is 'Haha' throw Error 'Wrong type'

or is it better to use instanceof:

haha = new Haha() unless haha instanceof Haha throw Error 'Wrong type'

One argument I have for instanceof is when using extends:

class BigHaha extends Haha bigHaha = new BigHaha console.log bigHaha instanceof Haha #true

but how safe is it, being a JavaScript operator - I feel like I should be sceptical about it.

On the other hand, with constructor.name it is very clear what is happening. Is it guaranteed that constructor.name will be set on all objects?

Thanks for any info.

mu is too short · Accepted Answer · 2014-04-02 19:37:21Z

17

First of all, constructor is also straight JavaScript:

Returns a reference to the Object function that created the instance's prototype.

So when you say o.constructor, you're really doing straight JavaScript, the name constructor for the CoffeeScript object initialization function is a separate matter.

So now you have a choice between using JavaScript's constructor property or JavaScript's instanceof operator. The constructor just tells you what "class" was used to create the object, instanceof on the other hand:

[...] tests whether an object has in its prototype chain the prototype property of a constructor.

So instanceof is the right choice if you want to allow for subclassing.

edited Apr 2, 2014 at 19:37

answered Jul 27, 2012 at 15:52

mu is too short

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8 Comments

phenomnomnominal Over a year ago

Great thanks! Just to confirm though, the name property on the constructor is CoffeeScript yes? And is reliable for classes where I don't care about sub/superclasses?

mu is too short Over a year ago

@phenomnomnominal: The name comes from JavaScript's Function and is non-standard so it may or may not work. Use instanceof or compare the constructor function directly. But really, just use instanceof.

Linus Thiel Over a year ago

I recall a discussion in the coffeescript issues about the constructor.name property being mangled by closure/uglifyjs. Another good thing to know is that instanceof is only really safe to use for your own objects, and not the built-in types. But I agree, for this use, instanceof is the better choice. Edit: here is a similar discussion.

mu is too short Over a year ago

@LinusGThiel: I could see running into name problems in all sorts of ways, especially if you're mixing JS and CS (M = Backbone.Model.extend({}), for example, is anonymous). And yes, all bets are off with native types.

phenomnomnominal Over a year ago

I've been using underscore.js for native type-checking - would you consider instanceof for my own classes and _.isWhatever for native objects to be fairly robust?

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