Set polyfill says it is [object Object] but should say [object Set]

[lll000111]

Environment

Any - you can reproduce the issue using only file

https://github.com/facebook/react-native/blob/master/Libraries/vendor/core/Set.js

and any Javascript encironment.

EDIT: Just for the bot, it is completely irrelevant:

Environment:
OS: Windows 10
Node: 10.3.0
Yarn: Not Found
npm: 6.1.0
Watchman: Not Found
Xcode: N/A
Android Studio: Version 3.1.0.0 AI-173.4720617

Packages: (wanted => installed)
react: 16.3.1 => 16.3.1
react-native: 0.55.4 => 0.55.4

Description and Steps to Reproduce

Here is code that creates a simple Set object and then uses the spread operator to spread the Set's contents into a new array.

First, using an environment where the above file was executed/loaded (plus its own dependencies), and I also short-circuited the detection at the beginning that determines whether the polyfill is applied:

const s1 = new Set([1,2,3]);
console.log(Object.prototype.toString.call(s1));

Result:

[object Object]

Expected Behavior

Now for comparison a clean ES6+ environment without anything loaded, so we get a native Set implementation:

const s1 = new Set([1,2,3]);
console.log(Object.prototype.toString.call(s1));

Result:

[object Set]

[lll000111]

Correction (edited main comment: When you test object spread directly it works, but I quote @G-2-Z from somewhere else:

The root cause is that the react-native JavaScript runtime uses a polyfill for Set that doesn't correctly handle calls of Object.prototype.toString.call(mySet). Therefore, testing [...mySet] works as expected but because Object.prototype.toString.call(mySet) returns '[object Object]' instead of '[object Set]' it is never called.

We use a custom JSON-stringifier (deterministic and has support for Map and Set) which uses an object's self-declared type to select the proper stringification, but that's just our anecdotal background story — in any case, the object should identify itself correctly.

[react-native-bot]

It looks like your issue may be missing some necessary information. Can you run react-native info and edit your issue to include these results under the Environment section?

[lll000111]

@react-native-bot Environment provided (and it still is unnecessary), now remove the label, please, bot....

[stale]

Hey there, it looks like there has been no activity on this issue recently. Has the issue been fixed, or does it still require the community's attention? This issue may be closed if no further activity occurs. You may also label this issue as "For Discussion" or "Good first issue" and I will leave it open. Thank you for your contributions.

[lll000111]

Here is what needs to be done: add a method on Symbol.toStringTag to an object that returns the name, here "Map" or "Set".

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/toStringTag

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/@@toStringTag

https://tc39.github.io/ecma262/#sec-object.prototype.tostring ("19.1.3.6 Object.prototype.toString ( )" item 15 "Let tag be ? Get(O, @@toStringTag).")

[lll000111]

To solve this, this is all that's needed:

    Object.defineProperty(
        Map.prototype,
        Symbol.toStringTag,
        {
            writable: false,
            enumerable: false,
            configurable: true,
            value: 'Map'
        }
    );

    Object.defineProperty(
        Set.prototype,
        Symbol.toStringTag,
        {
            writable: false,
            enumerable: false,
            configurable: true,
            value: 'Set'
        }
    );

You can read more at http://2ality.com/2015/09/well-known-symbols-es6.html#overriding-the-default-tostring-tag