IE11: Enumerating Object properties has Symbol polyfill keys in them

[petetnt]

Sorry in advance if this actually has nothing to do with Closure Compiler directly. I'd assume that it's done on purpose (or not?) and mostly looking for alternative ways to prevent the issue.

We have been debugging some issues caused by Google Maps version >3.29 on Internet Explorer 11 for a while now and finally most likely found a root cause in

apollographql/apollo-client#3043 (comment)

TL;DR: Recursively enumerating Object properties causes endless loops (or at least really really deep recursion) on IE11 because Google Maps now uses a version of Symbol polyfill that adds the keys to object. Maybe related to #2448 or similar?

Steps to reproduce:

Option 1:

• Use the Apollo repo linked in the issue

Option 2:

• Add a blank page with Google Maps v3.30 embedded
• Inline some large-ish object
• deepFreeze

It's hard to say what happened between the versions v3.29 and v3.30 that caused the breakage (versions updated, compiler flags changed) but hopefully you Googlers can check the menorepo or ask across the table or something (😂) to find out more.

[petetnt]

I assume this is the commit which caused the regression? 83857a5#diff-d22d8209fc94aff372133012210c694c

thoughts @shicks @blickly? I guess this can be worked around by checking if the symbol is polyfilled with typeof Symbol('') !== 'string'

[brad4d]

See also related issue #3052

There the problem is that libraries are using typeof x to determine whether x is a string or a symbol, and that doesn't work properly when the symbol comes from our polyfill.

[brad4d]

Just left a suggested workaround on #3052.
If you're already using a JS library that provides a better Symbol polyfill than the one provided by closure-compiler, you may be able to resolve this problem by just making sure that library loads before the Maps API (or any other JS code compiled with closure-compiler).

This will ensure that the closure-compiler polyfill code sees an already-polyfilled Symbol and does not try to add it's own polyfill for it.

[concavelenz]

Generally, polyfilling "WeakMap" or "symbol" is going to be problematic regardless of how you do it. I think the best solution would be to ask the Maps team to be careful not to use these in their API library. Using any polyfill on a page you don't own is generally problematic.