Issue: Cannot assign to read only property 'length' of function

[Obi-Dann]

We have a problem happening with 92 users (it's way less than 1% of our users)

The error is Cannot assign to read only property 'length' of function
at IObservableFactories line 1935, column 30 (webpack:///./node_modules/mobx/lib/mobx.js:1935)

It maps to line 195 in the following snippet

mobx/src/api/observable.ts

Lines 191 to 195 in b7fef45

	// weird trick to keep our typings nicely with our funcs, and still extend the observable function
	// ES6 class methods aren't enumerable, can't use Object.keys
	Object.getOwnPropertyNames(IObservableFactories.prototype)
	.filter(name => name !== "constructor")
	.forEach(name => (observable[name] = IObservableFactories.prototype[name]))

Somehow, IObservableFactories.prototype have a property called length. Attempting to assign observable['length'] fails in strict mode, because length is a read-only property.

I found no way to reproduce it. I am really keen on blaming browser extensions, but this issue happens across multiple browsers:

4 browsers affected	%
Chrome	61%
Mobile Safari	35%
IE	2%
Edge	2%

I guess the only idea I have is be wrap the code assigning props into try{} catch(e){} or to check filter out non-writable properties.

I don't mind creating a PR, but it'd be nice to know what you guys think and what approach is preferable before I do it

[Obi-Dann]

Perhaps, the PR I posted is not going to solve the issue, we've just had another similar error report in environment with the proposed fix applied

Going to investigate deeper

[Obi-Dann]

Weird, IObservableFactories.prototype must have a value of some kind of rubbish object but it does not make sense how that value gets assigned...

[mweststrate]

@DanNSam I would be fine by introducing a simple workaround, that filters out length as well, or just make everythign explicit, kill the loop, and copy the relevant functions by hand. Interested in changing your PR to that if that solves the issue?

[Obi-Dann]

Thanks @mweststrate I'll check it out and change my PR if it solves the issue!

[Obi-Dann]

I am going to trial these changes df5002c over the weekend and create a PR if the issue is resolved

[Obi-Dann]

Trialed changes from #1257 over the weekend. There were no new error reports, I think it fixes the issue. @mweststrate, could you please review and merge #1257 if it looks good to you? I wonder if it's acceptable to remove IObservableProperties from API

The only reason, I can think of, why the issue originally occurred is a browser extension polyfilling/mutating Object. getOwnPropertyNames, but it's a wild guess.

[mweststrate]

Released as 3.3.3. Thanks for the investigation and PR!