Faster/simpler isPlainObject check

[timdorr]

This runs up the prototype chain to check equivalency. When run in a cross-realm environment (differing contexts, iframes, etc), this ensures we're checking the value's prototype matches its context-specific instance of Object.

This is faster than calling toString() on the constructor. There's still the baseGetTag() call, which does its own toString(), but this swaps out one for cross-realm stuff. It's also a bit simpler to understand, I think.

[jsf-clabot]

All committers have signed the CLA.

[timdorr]

Do I need to do anything to get the tests to run? I checked out the 3203 branch and ran them there successfully.

[jdalton]

Do I need to do anything to get the tests to run? I checked out the 3203 branch and ran them there successfully.

Awesome! Thank you!

That's fine for now. I think we can even remove some of the baseTag checks too.

[timdorr]

I think we can even remove some of the baseTag checks too.

I had some difficulty there. These two tests fail in that case:

should return `false` for non-Object objects
should return `false` for objects with a read-only `Symbol.toStringTag` property

There's some edge case handling in #2705, but that's a good issue to read through for a reminder of why this check exists.

It does feel a little weird to be looking to toString() to answer the question. I'm not sure if there's a workaround, but with all these new reflective APIs to override the toString() output, it's definitely a complex problem.

[jdalton]

Cool. I'll re-evaluate those checks for v5.

[lock]

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