JavaScript Language feature Idea

Michael Theriot michael.lee.theriot at gmail.com
Mon Apr 18 19:31:55 UTC 2016


This can be trivially done with proxies. I don't think arrays should have a
special trap dependent on whether or not a symbol is set. If I were to make
a custom array with proxies I would then have to check if someone changed
standard behavior by inserting a symbol, and pass the symbol to the target
just to work properly.

If the `var arr = [1,2,3];` syntax is the real reason to change default
behavior then just use `Array.from` like such `var arr =
InvertedArray.from([1,2,3]);`. See below...

```
var getProp = (target, property) => typeof property !== 'symbol' &&
target.length && property < 0 && property >= -target.length ? +property +
target.length : property;

var handler = {
  get: (target, property, receiver) => Reflect.get(target, getProp(target,
property), receiver),
  set: (target, property, value, receiver) => Reflect.set(target,
getProp(target, property), value, receiver)
};

var InvertedArray = new Proxy(function InvertedArray(arg1) {}, {
  construct: (target, arguments, newTarget) => new
Proxy(Reflect.construct(Array, arguments, InvertedArray), handler)
});

Reflect.setPrototypeOf(InvertedArray, Array);

InvertedArray.prototype = Object.create(Array.prototype);
```

Arrays are effectively regular objects with traps on get/set to update the
length property. I don't think they should have a special new syntax to
access negative indices like `arr[:-1]` since it would logically have to
work on all objects and is dependent on a length property; it's just
syntactic sugar for yet another trap.

On Mon, Apr 18, 2016 at 2:05 PM, kdex <kdex at kdex.de> wrote:

> `Symbol.implementation` should be fairly trivial to implement once [Realm](
> https://github.com/caridy/proposal-realms)s are around, without affecting
> global scope.
>
> On Montag, 18. April 2016 18:58:06 CEST Andy Earnshaw wrote:
> > I don't think that would be trivial to implement and there might not be a
> > common enough use case for it.  You might want to look into something
> like
> > http://sweetjs.org if it's the syntactic sugar you're looking for.
> >
> > On Mon, 18 Apr 2016 19:35 /#!/JoePea, <joe at trusktr.io> wrote:
> >
> > > > ```js
> > > > Array[Symbol.implementation] = MyArray;
> > > > ```
> > >
> > > > That would mean all other programs executing on the page would be
> forced
> > > to use that Array implementation
> > >
> > > And also with my suggestion that would impact all code too.
> > >
> > > Would it be possible to limit the effect of using certain symbols to a
> > > scope where the symbol is used? For example:
> > >
> > > ```js
> > > function main() {
> > >   Array[Symbol.implementation] = MyArray;
> > >
> > >   let a = [1,2,3] // uses MyArray
> > > }
> > > let a = [1,2,3] // uses Array
> > > main()
> > > ```
> > >
> > > or
> > >
> > > ```js
> > > Array[Symbol.implementation] = MyArray;
> > > function main() {
> > >   let a = [1,2,3] // uses MyArray, from outer scope
> > > }
> > > let a = [1,2,3] // uses MyArray
> > > main()
> > > ```
> > >
> > > Or maybe some other method on a per-scope basis?
> > >
> > > On Mon, Apr 18, 2016 at 11:25 AM, Andy Earnshaw <
> andyearnshaw at gmail.com>
> > > wrote:
> > > > That would mean all other programs executing on the page would be
> forced
> > > to
> > > > use that Array implementation, imposing potentially critical problems
> > > with,
> > > > for example, performance and expected behavior. It's just not a good
> > > idea.
> > > >
> > > > I missed off esdiscuss when I replied earlier, but I mentioned that
> the
> > > only
> > > > reasonable solution is to introduce new syntax, e.g.
> > > >
> > > >     myArray[:-1]
> > > >
> > > > However, it's been said that there needs to be a compelling reason
> to add
> > > > new syntax and I'm not sure this qualifies imo.
> > > >
> > > >
> > > > On Mon, 18 Apr 2016 19:11 kdex, <kdex at kdex.de> wrote:
> > > >>
> > > >> Yes, now we're heading in the right direction.
> > > >>
> > > >> The problem with something like `Symbol.propertyAccess` is that this
> > > might
> > > >> lead to a flood of new well-known Symbols.
> > > >> Conceptually, `Symbol.propertyAccess` sounds like it should have
> been a
> > > >> `Proxy` trap, anyway.
> > > >>
> > > >> Here's an more general idea: Why not allow users to set a derived
> class
> > > >> for literals via well-known Symbols?
> > > >> Thus, users could provide custom implementations for `RegExp`,
> `Array`,
> > > >> `Object` (…) literals, as long as the value points to a derived
> class.
> > > >>
> > > >> We could even introduce negative array indices in a way that doesn't
> > > break
> > > >> the web like this:
> > > >>
> > > >> ```js
> > > >> [1, 2, 3][-1]; // undefined
> > > >> Array[Symbol.implementation] = MyArray;
> > > >> [1, 2, 3][-1]; // 3
> > > >> Array[Symbol.implementation] = 3; // TypeError: Array
> implementations
> > > must
> > > >> extend Array (→ Array.isPrototypeOf(Number(3)) is false)
> > > >> ```
> > > >>
> > > >> On Montag, 18. April 2016 10:47:24 CEST /#!/JoePea wrote:
> > > >> > But, can
> > > >> >
> > > >> > ```js
> > > >> > let a = [1,2,3]
> > > >> > ```
> > > >> >
> > > >> > create a new MyArray? Maybe, instead of having negative indices by
> > > >> > default (which breaks some backwards compatibility) we can
> introduce a
> > > >> > symbol for overriding property access? Something like
> > > >> >
> > > >> > ```js
> > > >> > Array.prototype[Symbol.propertyAccess] = function(index) {
> > > >> >   if (index < 0) ...
> > > >> >   else ...
> > > >> > }
> > > >> > ```
> > > >> >
> > > >> > ? Just an idea; I'm not sure if that's a good use for Symbols. We
> > > >> > could then easily add this helper code to a given app.
> > > >> >
> > > >> > On Mon, Apr 18, 2016 at 10:25 AM, kdex <kdex at kdex.de> wrote:
> > > >> > > I don't see a good reason why to mangle with this.
> > > >> > > Note that you can achieve this behavior without breaking
> backwards
> > > >> > > compatibility with ES6 Proxies:
> > > >> > >
> > > >> > > ```js
> > > >> > > class MyArray extends Array {
> > > >> > >         constructor(...args) {
> > > >> > >                 super(...args);
> > > >> > >                 function computeProperty(target, property) {
> > > >> > >                         const index = +property;
> > > >> > >                         return index < 0 ? String(target.length
> +
> > > >> > > index) : property;
> > > >> > >                 }
> > > >> > >                 return new Proxy(this, {
> > > >> > >                         get(target, property, receiver) {
> > > >> > >                                 return Reflect.get(target,
> > > >> > > computeProperty(target, property), receiver);
> > > >> > >                         },
> > > >> > >                         set(target, property, receiver) {
> > > >> > >                                 return Reflect.set(target,
> > > >> > > computeProperty(target, property), receiver);
> > > >> > >                         }
> > > >> > >                 });
> > > >> > >         }
> > > >> > > }
> > > >> > > ```
> > > >> > >
> > > >> > > On Montag, 18. April 2016 09:59:15 CEST /#!/JoePea wrote:
> > > >> > >> Backwards compatibility has been broken before. I don't think
> this
> > > >> > >> one
> > > >> > >> is too bad of a breakage.
> > > >> > >>
> > > >> > >> On Sun, Apr 17, 2016 at 9:48 PM, Biju <bijumaillist at gmail.com>
> > > wrote:
> > > >> > >> > On 17 April 2016 at 17:29, Frankie Bagnardi <
> > > f.bagnardi at gmail.com>
> > > >> > >> > wrote:
> > > >> > >> >> That would break backward compatibility;
> > > >> > >> >>
> > > >> > >> >> ```js
> > > >> > >> >> var a = ['a'];
> > > >> > >> >> a['-1'] = 'test';
> > > >> > >> >> Object.keys(a) // ['0', '-1']
> > > >> > >> >> ```
> > > >> > >> >
> > > >> > >> > Do we have statistics how many sties depend on that?
> > > >> > >> > _______________________________________________
> > > >> > >> > es-discuss mailing list
> > > >> > >> > es-discuss at mozilla.org
> > > >> > >> > https://mail.mozilla.org/listinfo/es-discuss
> > > >> > >> _______________________________________________
> > > >> > >> es-discuss mailing list
> > > >> > >> es-discuss at mozilla.org
> > > >> > >> https://mail.mozilla.org/listinfo/es-discuss
> > > >> > >>
> > > >> > > _______________________________________________
> > > >> > > es-discuss mailing list
> > > >> > > es-discuss at mozilla.org
> > > >> > > https://mail.mozilla.org/listinfo/es-discuss
> > > >> >
> > > >> _______________________________________________
> > > >> es-discuss mailing list
> > > >> es-discuss at mozilla.org
> > > >> https://mail.mozilla.org/listinfo/es-discuss
> > >
> >
> _______________________________________________
> es-discuss mailing list
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>
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