> `o.foo(); // works` I think there is something missing from that example as that line throws before it can get to the `new Proxy` line.
#!/JoePea On Wed, Jul 15, 2020 at 10:47 PM Jordan Harband <[email protected]> wrote: > > So can: > ```jsx > const o = { foo() { if (o.foo !== this) { throw 'detected'; } } }; > o.foo(); // works > new Proxy(o, {}).foo(); // throws > ``` > > (as would a class that used a closed-over WeakMap for each "private field") > > Private fields do not introduce any new hazards here. > > On Tue, Jul 14, 2020 at 8:18 PM #!/JoePea <[email protected]> wrote: >> >> > private members (safely) allow classes with internal slots. >> >> I'd say that they aren't safe, if they can break 3rd-party code on the >> external public side. >> >> #!/JoePea >> >> On Sun, Jul 12, 2020 at 3:09 PM Michael Theriot >> <[email protected]> wrote: >> > >> > I assume OP wants to use proxies and private members together. They are >> > not designed to be compatible. >> > >> > Proxies and private members are a UX goal primarily for developers. >> > Proxies easily allow observation of another object or creation of exotic >> > objects (e.g. Array), and private members (safely) allow classes with >> > internal slots. Since they cannot be used together the issue exists, and >> > the hack circumvents this by reimplementing private in a way that does not >> > require private fields. >> > >> > On Sun, Jul 12, 2020 at 4:45 PM kai zhu <[email protected]> wrote: >> >> >> >> as product-developer, can i ask what ux-objective you ultimately want >> >> achieved? >> >> >> >> ```js >> >> const sub = new Sub() >> >> >> >> // i'm a noob on proxies. what is this thing (with >> >> proxied-private-fields) ultimately used for? >> >> const proxy = new Proxy(sub, ...) >> >> ``` >> >> >> >> On Sun, Jul 12, 2020 at 4:34 PM Michael Theriot >> >> <[email protected]> wrote: >> >>> >> >>> This does require you to have both the key and the weakmap though, so it >> >>> actually does succeed in hiding the data so long as the weakmap is out >> >>> of scope. I guess the issue I can foresee is that the key could be >> >>> modified after the object is created. >> >>> >> >>> e.g. >> >>> ```js >> >>> var a = new A(); >> >>> var key = Object.getOwnPropertySymbols(a)[0]; >> >>> delete a[key]; >> >>> a.hidden; // throws >> >>> ``` >> >>> >> >>> That itself can be guarded by just making the key undeletable. So, I >> >>> guess this solution could work depending what your goals are? >> >>> >> >>> On Sun, Jul 12, 2020 at 4:21 PM Michael Theriot >> >>> <[email protected]> wrote: >> >>>> >> >>>> It nearly works, but the issue is that the key will be leaked by >> >>>> `Object.getOwnPropertySymbols(new A())`, so it's not truly private. >> >>>> >> >>>> There have been ideas proposing "private symbols" but I am not familiar >> >>>> with their issues, and I would guess with Class Fields they are >> >>>> unlikely to materialize anyway. >> >>>> >> >>>> On Sun, Jul 12, 2020 at 2:19 PM François REMY >> >>>> <[email protected]> wrote: >> >>>>> >> >>>>> At the risk of pointing out the obvious: >> >>>>> >> >>>>> >> >>>>> >> >>>>> ```js >> >>>>> >> >>>>> const privkey = Symbol(); >> >>>>> >> >>>>> const stores = new WeakMap(); >> >>>>> >> >>>>> >> >>>>> >> >>>>> class A { >> >>>>> >> >>>>> [privkey] = {}; >> >>>>> >> >>>>> constructor() { >> >>>>> >> >>>>> const priv = {}; >> >>>>> >> >>>>> priv.hidden = Math.random(); >> >>>>> >> >>>>> stores.set(this[privkey], priv); >> >>>>> >> >>>>> } >> >>>>> >> >>>>> >> >>>>> >> >>>>> get hidden() { >> >>>>> >> >>>>> const priv = stores.get(this[privkey]); >> >>>>> >> >>>>> return priv.hidden; >> >>>>> >> >>>>> } >> >>>>> >> >>>>> } >> >>>>> >> >>>>> >> >>>>> >> >>>>> var as = [ >> >>>>> >> >>>>> new A(), >> >>>>> >> >>>>> new Proxy(new A(),{}), >> >>>>> >> >>>>> new Proxy(new A(),{}), >> >>>>> >> >>>>> ]; >> >>>>> >> >>>>> console.log(as.map(a=>a.hidden)); >> >>>>> >> >>>>> ``` >> >>>>> >> >>>>> >> >>>>> >> >>>>> >> >>>>> >> >>>>> >> >>>>> >> >>>>> From: Michael Theriot >> >>>>> Sent: Sunday, July 12, 2020 20:59 >> >>>>> To: Michael Haufe >> >>>>> Cc: [email protected] >> >>>>> Subject: Re: Why does a JavaScript class getter for a private field >> >>>>> fail using a Proxy? >> >>>>> >> >>>>> >> >>>>> >> >>>>> I experienced this issue prior to this proposal, using weakmaps for >> >>>>> private access. >> >>>>> >> >>>>> >> >>>>> >> >>>>> e.g. >> >>>>> >> >>>>> ```js >> >>>>> >> >>>>> const stores = new WeakMap(); >> >>>>> >> >>>>> class A { >> >>>>> constructor() { >> >>>>> const priv = {}; >> >>>>> priv.hidden = 0; >> >>>>> stores.set(this, priv); >> >>>>> } >> >>>>> >> >>>>> get hidden() { >> >>>>> const priv = stores.get(this); >> >>>>> return priv.hidden; >> >>>>> } >> >>>>> } >> >>>>> >> >>>>> const a = new A(); >> >>>>> console.log(a.hidden); // 0 >> >>>>> >> >>>>> const p = new Proxy(a, {}); >> >>>>> console.log(p.hidden); // throws! >> >>>>> >> >>>>> ``` >> >>>>> >> >>>>> >> >>>>> >> >>>>> I found a workaround: >> >>>>> >> >>>>> >> >>>>> >> >>>>> ```js >> >>>>> const stores = new WeakMap(); >> >>>>> >> >>>>> class A { >> >>>>> constructor() { >> >>>>> const priv = {}; >> >>>>> priv.hidden = 0; >> >>>>> stores.set(this, priv); >> >>>>> >> >>>>> const p = new Proxy(this, {}); >> >>>>> stores.set(p, priv); // set proxy to map to the same private store >> >>>>> >> >>>>> return p; >> >>>>> } >> >>>>> >> >>>>> get hidden() { >> >>>>> const priv = stores.get(this); // the original instance and proxy >> >>>>> both map to the same private store now >> >>>>> return priv.hidden; >> >>>>> } >> >>>>> } >> >>>>> >> >>>>> const a = new A(); >> >>>>> >> >>>>> console.log(a.hidden); >> >>>>> ``` >> >>>>> >> >>>>> >> >>>>> >> >>>>> Not ideal, and only works if you provide the proxy in the first place >> >>>>> (e.g. making exotic JS objects). But, not necessarily a new issue with >> >>>>> proxies, either. >> >>>>> >> >>>>> >> >>>>> >> >>>>> On Fri, Jun 5, 2020 at 12:29 AM Michael Haufe >> >>>>> <[email protected]> wrote: >> >>>>> >> >>>>> This is a known issue and very painful for me as well. You can see a >> >>>>> long ugly discussion here: >> >>>>> >> >>>>> >> >>>>> >> >>>>> <https://github.com/tc39/proposal-class-fields/issues/106> >> >>>>> >> >>>>> >> >>>>> >> >>>>> I suggest the following guide to assist you: >> >>>>> >> >>>>> >> >>>>> >> >>>>> <https://javascript.info/proxy#proxy-limitations> >> >>>>> >> >>>>> >> >>>>> >> >>>>> Another possible approach is to have your classes extend a proxy: >> >>>>> >> >>>>> >> >>>>> >> >>>>> <https://github.com/tc39/proposal-class-fields/issues/106#issuecomment-397484713> >> >>>>> >> >>>>> >> >>>>> >> >>>>> >> >>>>> >> >>>>> From: es-discuss <[email protected]> On Behalf Of Laurie >> >>>>> Harper >> >>>>> Sent: Friday, June 5, 2020 12:21 AM >> >>>>> To: [email protected] >> >>>>> Subject: Why does a JavaScript class getter for a private field fail >> >>>>> using a Proxy? >> >>>>> >> >>>>> >> >>>>> >> >>>>> I can expose private class fields in JavaScript using getters, and >> >>>>> those getters work correctly when invoked on instances of a subclass. >> >>>>> However, if I then wrap the instance with a proxy the getter will >> >>>>> throw a type error, even if the proxy `get` hook uses `Reflect.get()`: >> >>>>> >> >>>>> ``` >> >>>>> class Base { >> >>>>> _attrA >> >>>>> #_attrB >> >>>>> >> >>>>> constructor() { >> >>>>> this._attrA = 100 >> >>>>> this.#_attrB = 200 >> >>>>> } >> >>>>> >> >>>>> get A() { return this._attrA } >> >>>>> >> >>>>> get B() { return this.#_attrB } >> >>>>> >> >>>>> incrA() { this._attrA++ } >> >>>>> >> >>>>> incrB() { this.#_attrB++ } >> >>>>> } >> >>>>> >> >>>>> class Sub extends Base {} >> >>>>> >> >>>>> const sub = new Sub() >> >>>>> >> >>>>> const proxy = new Proxy(sub, { >> >>>>> get(target, prop, receiver) { >> >>>>> const value = Reflect.get(target, prop, receiver) >> >>>>> return typeof value === 'function' ? value.bind(target) : >> >>>>> value // (1) >> >>>>> } >> >>>>> }) >> >>>>> >> >>>>> console.log('sub.A', sub.A) // OK: -> 100 >> >>>>> console.log('sub.B', sub.B) // OK: -> 200 >> >>>>> sub.incrA() // OK >> >>>>> sub.incrB() // OK >> >>>>> console.log('sub.A', sub.A) // OK: -> 101 >> >>>>> console.log('sub.B', sub.B) // OK: -> 201 >> >>>>> >> >>>>> console.log('proxy.A', proxy.A) // OK: -> 100 >> >>>>> console.log('proxy.B', proxy.B) // TypeError: Cannot read private >> >>>>> member #_attrB from an object whose class did not declare it >> >>>>> proxy.incrA() // OK >> >>>>> proxy.incrB() // OK due to (1) >> >>>>> console.log('proxy.A', proxy.A) // OK: -> 100 >> >>>>> console.log('proxy.B', proxy.B) // TypeError: Cannot read private >> >>>>> member #_attrB from an object whose class did not declare it >> >>>>> ``` >> >>>>> >> >>>>> The call to `proxy.incrB()` works, because the proxy handler >> >>>>> explicitly binds function values to `target` on line (1). Without the >> >>>>> `bind()` call, the `proxy.incrB()` invocation would throw a >> >>>>> `TypeError` like the getter invocation does. That makes some sense: >> >>>>> the result of the call to `Reflect.get()` is the 'unbound' function >> >>>>> value of the property being retrieved, which must then be bound to >> >>>>> `target`; it would make more sense, though, if `this` binding was >> >>>>> applied by the [[Call]] operation on the result of the [[Get]] >> >>>>> operation... >> >>>>> >> >>>>> But there is no opportunity to 'bind' a getter before invoking it; as >> >>>>> a result, a proxied getter ends up receiving the wrong `this` binding, >> >>>>> leading to the inconsistency. >> >>>>> >> >>>>> Is there any way to make this work correctly? The only approach I can >> >>>>> think of (which I haven't tried) would be to have the `get` hook walk >> >>>>> up the prototype chain, starting from `target`, calling >> >>>>> `getOwnPropertyDescriptor()` and checking for a getter method, and >> >>>>> explicitly applying the getter with an adjusted `this` binding. That >> >>>>> sounds ludicrously cumbersome and brittle... >> >>>>> >> >>>>> Is there a better way to get this working correctly? >> >>>>> >> >>>>> >> >>>>> >> >>>>> -- >> >>>>> >> >>>>> Laurie >> >>>>> >> >>>>> _______________________________________________ >> >>>>> es-discuss mailing list >> >>>>> [email protected] >> >>>>> https://mail.mozilla.org/listinfo/es-discuss >> >>>>> >> >>>>> >> >>> >> >>> _______________________________________________ >> >>> es-discuss mailing list >> >>> [email protected] >> >>> https://mail.mozilla.org/listinfo/es-discuss >> > >> > _______________________________________________ >> > es-discuss mailing list >> > [email protected] >> > https://mail.mozilla.org/listinfo/es-discuss >> _______________________________________________ >> es-discuss mailing list >> [email protected] >> https://mail.mozilla.org/listinfo/es-discuss _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
