1644581 - Iterator.prototype[@@toStringTag] breaks web compatibility

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Adam Vandolder [:avandolder]]

Iterator.prototype[@@toStringTag] was created as non-writable in https://bugzilla.mozilla.org/show_bug.cgi?id=1640923, behind the javascript.options.experimental.iterator_helpers pref, which breaks web compatibility.

Comment 1

[Adam Vandolder [:avandolder]]

Attachment: Bug 1644581 - Remove Iterator.prototype[@@toStringTag]. r?jorendorff

Comment 2

[Adam Vandolder [:avandolder]]

https://treeherder.mozilla.org/#/jobs?repo=try&revision=cb644640a78dee681f8fbff523575356773fbfc4

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/4a3b83f31a88
Remove Iterator.prototype[@@toStringTag]. r=jorendorff

Comment 4

[Bogdan Tara[:bogdan_tara | bogdant]]

https://hg.mozilla.org/mozilla-central/rev/4a3b83f31a88

Comment 5

[ljharb]

Can someone elaborate on exactly what broke? ie, what makes it not web compatible?

Comment 6

[Adam Vandolder [:avandolder]]

Defining toStringTag on Iterator.prototype (specifically, defining it as non-writable) causes regenerator to throw Uncaught TypeError: Symbol.toStringTag is read-only here: https://github.com/facebook/regenerator/blob/master/packages/regenerator-runtime/runtime.js#L389

This causes sites like Slack and Discord to fail to render at all.

It could be that is an implementation issue specific to Firefox, I'm not 100% sure whether what regenerator does (polyfill Generators and set a toStringTag on their prototype) is legal under the spec or not.

Comment 7

[ExE Boss]

That is the correct behaviour of OrdinarySetWithOwnDescriptor when a parent has a non‑writable property.

Regenerator should be using Object.defineProperty instead.

Comment 8

[ljharb]

Alternatively, Regenerator should be doing if (Gp[toStringTagSymbol] !== "Generator") { Gp[toStringTagSymbol] = "Generator"; }.

Thanks, that explains the issue.

Comment 9

[Ben Newman]

The regenerator-runtime problem discussed above should now be fixed in regenerator-runtime@0.13.6, thanks to this PR from ljharb (with tests by ExE Boss): https://github.com/facebook/regenerator/pull/400. I'm the maintainer of Regenerator, by the way.