Implement in two browsers

[ljharb]

In order to progress to stage 4, this proposal must be implemented in two stable browsers, not behind any flags.

Implemented:

• Firefox / SpiderMonkey [bug]
• Chrome / V8 [patch]
• Safari + Webkit / JavaScriptCore [patch]
• Edge / Chakra [issue / PR]

At least behind a flag:

• Firefox / SpiderMonkey
• Chrome / V8
• Safari + Webkit / JavaScriptCore
• Edge / Chakra

Shipped stable:

• Firefox / SpiderMonkey
• Chrome / V8
• Safari + Webkit / JavaScriptCore
• Edge / Chakra

[WebReflection]

I think the Chromium patch covers the "Behind a flag" check too.

[ljharb]

Sure, I'll just check that off once it lands in Canary.

[mathiasbynens]

Nit: It’s V8, not “v8”.

[WebReflection]

For record sake, I've filed a Firefox bug/request: https://bugzilla.mozilla.org/show_bug.cgi?id=1245024

[ljharb]

@mathiasbynens fixed (altho that's sad, i really prefer the lowercase v aesthetically)

[caitp]

landed in WebKit last night: https://webkit.org/b/153799 / https://webkit.org/b/153814

[WebReflection]

thanks @caitp is it behind a flag or enabled by default?

[caitp]

It's on by default

[WebReflection]

👍

[ljharb]

@caitp Awesome!!

[ljharb]

Linking to tc39/ecma262#593 (comment) since this feature's spec changed slightly in July 2016.

[WebReflection]

anything the polyfill should consider? I'm not sure I understand undefined descriptors ... are those neither data nor accessors ?

[caitp]

@WebReflection EG, if Object.getOwnPropertyDescriptor returns undefined for a name, it is not added to the result object

[WebReflection]

I've got the idea behind, yet I don't know how that would be possible. Can you define a property without a descriptor? Any example in order to reproduce with a test and eventually update my code?

Thanks

[caitp]

You can (under some conditions) lie about the properties a Proxy has.

var p = new Proxy({}, { ownKeys() { return ["fakeProperty"]; } });
Object.getOwnPropertyDescriptors(p);

// Let ownKeys be ? obj.[[OwnPropertyKeys]]() -> ["fakeProperty"]
// Let desc be ? obj.[[GetOwnProperty]]("fakeProperty") -> undefined
// Let descriptor be ! FromPropertyDescriptor(desc). -> undefined
// ...

We don't throw an exception for lying about property keys in this case, because invariants are not violated, so it's possible to get an undefined descriptor into the algorithm.

[caitp]

or in a polyfill this might look like

let names = Reflect.ownKeys(p); // ["fakeProperty"]
for (let key of names) {
  // desc = p.[[GetOwnProperty]]("fakeProperty") -> undefined
  let desc = Reflect.getOwnPropertyDescriptor(p, key);
  // ...
}

[WebReflection]

got it, thanks

[ljharb]

@WebReflection the polyfill is updated in ljharb/Object.getOwnPropertyDescriptors@fc060fc - i updated the reference implementation in c13d24e as well (thanks for the reminder).