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Use a Symbol to tag every ReactElement
Fixes #3473 I tag each React element with `$$typeof: Symbol.for('react.element')`. We need this to be able to safely distinguish these from plain objects that might have come from user provided JSON. The idiomatic JavaScript way of tagging an object is for it to inherent some prototype and then use `instanceof` to test for it. However, this has limitations since it doesn't work with value types which require `typeof` checks. They also don't work across realms. Which is why there are alternative tag checks like `Array.isArray` or the `toStringTag`. Another problem is that different instances of React that might have been created not knowing about eachother. npm tends to make this kind of problem occur a lot. Additionally, it is our hope that ReactElement will one day be specified in terms of a "Value Type" style record instead of a plain Object. This Value Types proposal by @nikomatsakis is currently on hold but does satisfy all these requirements: https://github.com/nikomatsakis/typed-objects-explainer/blob/master/valuetypes.md#the-typeof-operator Additionally, there is already a system for coordinating tags across module systems and even realms in ES6. Namely using `Symbol.for`. Currently these objects are not able to transfer between Workers but there is nothing preventing that from being possible in the future. You could imagine even `Symbol.for` working across Worker boundaries. You could also build a system that coordinates Symbols and Value Types from server to client or through serialized forms. That's beyond the scope of React itself, and if it was built it seems like it would belong with the `Symbol` system. A system could override the `Symbol.for('react.element')` to return a plain yet cryptographically random or unique number. That would allow ReactElements to pass through JSON without risking the XSS issue. The fallback solution is a plain well-known number. This makes it unsafe with regard to the XSS issue described in #3473. We could have used a much more convoluted solution to protect against JSON specifically but that would require some kind of significant coordination, or change the check to do a `typeof element.$$typeof === 'function'` check which would not make it unique to React. It seems cleaner to just use a fixed number since the protection is just a secondary layer anyway. I'm not sure if this is the right tradeoff. In short, if you want the XSS protection, use a proper Symbol polyfill. Finally, the reason for calling it `$$typeof` is to avoid confusion with `.type` and the use case is to add a tag that the `typeof` operator would refer to. I would use `@@typeof` but that seems to deopt in JSC. I also don't use `__typeof` because this is more than a framework private. It should really be part of the polyfilling layer.
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