Why are ES6 class methods not automatically bound to the instance?
One of the wonderful features of many prototype methods is that they can be
borrowed and .call-ed on other objects (imagine if you couldn't
Array.prototype.slice.call(arguments)!). Auto binding would cripple this
feature, and it seems like opting out would be harder than opting in.
I'm not sure if that was part of the reasoning or not, but I believe it was definitely the right decision.
On Wed, Feb 10, 2016 at 7:11 AM, Sam Gluck <sdgluck at gmail.com> wrote:
Hi all,
Hope this is the right place for a purely out-of-interest query - if not, apologies, please ignore!
Why are methods on an ES6 class not automatically bound to their instance? Is there any public documentation/conversation, or reasoning, that can explain this decision? (I couldn't find any on esdiscuss.org.)
That is an excellent question. Several of the early class proposals did so, as they were starting with the semantics of es5 objects-as-closures and classes as compositions-of-instance-traits.
The idea was that language support would make this semantics efficient, avoiding the need to eagerly allocate a closure per method per instance.
However, for reasons I understand, these failed to gain traction. Instead, we moves towards sugar for the dominant es5 pattern of encoding classes into prototype inheritance. Initially, we tried to have this purely be sugar, so that people could painlessly refactor code in that dominant pattern into classes.
As we wrestled with the detailed semantics around super and construction, es6 classes deviated from being pure sugar. But this deviation only prevents painless refactoring from es6 classes into the dominant es5 pattern. Practically, it remains painless to refactor from the es5 pattern into es6 classes.
At zenparsing/es-function-bind#17 we realized
we could still have had methods bind on extraction -- accounting for the behavior by decreeing that methods are installed on the prototype as accessors whose getter binds. However, this realization came too late for es6. Since it would have made the refactoring into classes more hazardous -- more of a semantic change -- it is not clear it would have flown even if we had thought of it in time. Instead, under all variations of the decorator designs, one can write such a decorator so that decorated methods are bind-on-extraction, by explicitly creating this accessor property. However(!), if implemented as a user-land decorator, this has much worse performance than objects-as-closures!! Objects-as-closures have higher allocation cost when allocating the object.
jsperf.com/creating-stateful-objects
But are quite efficient at using the object once the object is created:
(Note that jsperf is misidentifying Edge 28.14257.1000.0 as Chrome 46.0.2486. This is worth noting because Edge uses the transposed representation for WeakMaps, and so WeakMap-based usage of private state has much less penalty on Edge. Though this is besides the point of this thread.)
To make a decorator for binding-on-extraction efficient, an implementation would need some kind of special case somewhere to avoid the allocation when the method is being immediately invoked, rather than being observably extracted. The only thing TC39 needs to do to enable this is to standardize such a decorator so that implementations can provide it as a builtin that they recognize.
this is an excellent point. I can't say I remember it being raised, but it would certainly have been adequate to kill any proposal that this be the default behavior.
I proposed a way to make method easily bindable in class definitions or object literals here: esdiscuss.org/topic/concise
Sorry the link was broke in my last email (the topic should be "concise-method-binding"): esdiscuss.org/topic/concise
Does anyone know why it keeps cutting off the end of my link?
One of the wonderful features of many prototype methods is that they can be borrowed and .call-ed on other objects (imagine if you couldn't
Array.prototype.slice.call(arguments)!). Auto binding would cripple this feature, and it seems like opting out would be harder than opting in.
In general, there is often a tension between making the language "better" (for some subjective value system) and maintaining consistency. I think maintaining consistency was the right call in this case.
Hope this is the right place for a purely out-of-interest query - if not, apologies, please ignore!
Why are methods on an ES6 class not automatically bound to their instance? Is there any public documentation/conversation, or reasoning, that can explain this decision? (I couldn't find any on esdiscuss.org.)
Thank you.
Best