Overriding Map/etc with get/set hooks?

Domenic Denicola domenic at domenicdenicola.com
Mon May 20 19:55:56 PDT 2013

Seems like this isn't really a Map? It'd be pretty confusing for something to pretend to be a Map but act in such coercive and side-effecty ways. Better to just obey the Map structural type, perhaps, to give people an interface they're used to while not pretending to be something you're not? 

> -----Original Message-----
> From: es-discuss-bounces at mozilla.org [mailto:es-discuss-
> bounces at mozilla.org] On Behalf Of Tab Atkins Jr.
> Sent: Monday, May 20, 2013 22:06
> To: es-discuss
> Subject: Overriding Map/etc with get/set hooks?
> For the CSS Variables spec I need to define an object with arbitrary string
> keys, with the initial set determined by the custom properties set in the style
> rule, and on modification I need to coerce the provided key to a string, and
> then go mutate the style rule accordingly.  When the style rule is mutated to
> add/remove custom properties, I also need to mutate the exposed keys on
> the object.  (In other words, this object has a bidirectional link with a style
> rule; it just exposes a more convenient and specialized interface for custom
> properties specifically.)
> Right now I'm defining this via the WebIDL getter/setter/etc hooks, which
> ends up defining an "object map".  This is bad practice, though, because
> anything set on the prototype chain will show up as a
> (non-own) key, potentially causing confusion.
> I'd like to convert this over to an ES Map, as that avoids the above issue and
> gets me all the Map extras for free, which is nice.
> However, I don't think it's currently possible to do what I need.
> Is it possible add appropriate hooks to the ES spec to let me define the
> [[MapData]] via a spec, rather than as an initially-empty list of tuples that
> get/set unconditionally read from?  I need to be able to define in spec-ese
> that the [[MapData]] tuples consist of some list of data from a style rule, and
> that whenever a value gets set, I first coerce the key to a string, and then go
> mutate the style rule instead of the [[MapData]] (it then picks up the
> mutated data by virtue of being defined by the style rule).
> (One way to do this today is to subclass Map and provide my own
> get/set/etc. functions, but I need to override a potentially-open set
> (anything that doesn't directly lean on my overridden functions), and it
> doesn't prevent people from directly twiddling my [[MapData]] by calling
> Map.prototype.set.call() on my object.)
> Alternately: Proxies?  Is that possible?  What benefits/drawbacks come from
> that?
> ~TJ
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