classes and enumerability

Glen Huang curvedmark at
Wed Dec 24 18:21:42 PST 2014

Just my 2 cents:

1. Rick’s field test approach is epic. Encourage me to get my hands dirty. :)

   However, the conclusion I get from these tests is that not many developers explicitly set things to be non-enumerable. This does not directly translate to “many developers care about prototype properties being enumerable”.

   I agree with Andrea here that `hasOwnProperty` has to be taken into consideration. Especially whether people use `hasOwnProperty` inside “for in”. Because that’s what enumerability of class syntax will eventually affect. Nothing else matters if I’m not wrong. (This seems to also be Allen’s opinion)

2. Allen’s study addressed exactly that.

   Especially the "ENUMERATION WITH FOR…IN” chapter. And again, epic. :)

   The conclusion in the study is that not many people use “hasOwnProperty” with “for in”. However, one thing I think is still missing is to examine that what kinds of objects are being used with “for in”. For example, if this is how they use “for in”:

var defaults = { … };
for (var key in defaults) { … }


var defaults = { … };
var options = Object.create(defaults);
for (var key in options) { … }

Then it should be totally fine if they don’t use “hasOwnProperty” and they don’t adding things to Object.prototype. (I think whether they (or the libs they use) add properties to Object.prototype should also be examined in this case).

Also “for in” over DOM objects should also be examined. Because enumerability of class syntax doesn’t matter here even if they don’t use “hasOwnProperty”.

So it pretty much comes down to this:

Do many developers use “for in” over objects created by user created constructors and don’t use “hasOwnProperty” when they do that.

If the answer is “yes”, then default class syntax to non-enum is probably not a good idea. Otherwise, it should be fine.

I will do some tests to see if I can answer that question with some concrete data. (probably is going to take some time, I don’t see reliable way of testing that right now). 

> On Dec 25, 2014, at 8:56 AM, Brendan Eich <brendan at> wrote:
> Sorry, you can't use small-world reasoning about programming in the large, especially with JS on the web. for-in proliferated, before hasOwnProperty appeared in ES3. Even after, the burden was high enough, per that study Allen cited ( for-in ain't going away.
> If you are saying we should make class prototype methods non-enumerable in ES6, I agree. If you're saying it won't matter that they remain as drafted, enumerable, because everyone will stop using for-in. I don't buy it. If you are suggesting that for-in-based code won't be run on class prototypes, perhaps -- but that's a gamble.
> /be
> Gary Guo wrote:
>> Actually I believe that there will not be many cases that we need to use for-in loop in ES6. In most cases enumerate through properties in the prototype chain is not preferable. for-of Object.keys should be enough for enumeration with own properties. If my assumption is correct, since we are less likely to use legacy ways, we will be more able to make a change. 
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