core-js WeakMaps are broken on React Native?

[gaearon]

As reported here by @davodesign84 and here, it appears that #45 broke React Native because for some reason WeakMap polyfill from core-js returns undefined:

undefined is not a constructor (evaluating 'new _weakMap2.default()')

Alas, I don’t have React Native at hand and can’t look into why this happens. We just use it like this:

import WeakMap from 'core-js/library/es6/weak-map';

I’ll be grateful if anybody can look into why this fails on RN. Possible alternatives:

• Use a different WeakMap polyfill
• Use a Map polyfill instead of the issue is specific to WeakMap

[gaearon]

@davodesign84 Are you up for helping me test different fixes?

[gaearon]

I’m trying to install React Native but it’s been 30 minutes and I have no idea what the progress is because there is no spinner. :-/

[davide-scalzo]

Sure, I can try!

On 6 Mar 2016, at 14:35, Dan Abramov notifications@github.com wrote:

I’m trying to install React Native but it’s been 30 minutes and I have no idea what the progress is because there is no spinner. :-/

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Reply to this email directly or view it on GitHub.

[gaearon]

Can you put console.log(_weakMap2) before the failing line and check what’s there?

[gaearon]

So far I haven’t been able to reproduce this with React Native 0.21.

• Are you using React Native as is, or via something like react-native-webpack-server?
• Are you using it with Parse as described in http://stackoverflow.com/questions/35822104/react-native-weakmap2-error?

[davide-scalzo]

Yes I am.

Here is what comes out with the console debug:

createClassProxy.js:53Object {default: undefined}

[gaearon]

OK, I repro’d it after including Parse. 👍
Let me look into this.

[gaearon]

Apparently the version we import stops working if some other package (in this case, Parse) imports WeakMap from core-js in babel-runtime@5.x. Not sure how to fix this.

[satya164]

I don't think we include babel-polyfill in React Native. So WeakMap might not be there,

[gaearon]

It’s not but I’m directly importing core-js polyfill. So I’m not relying on RN shipping it. It works fine unless some other package also imports core-js polyfill at which point it stops working.

[satya164]

@gaearon Ah. That makes sense. Is it possible that the Babel transforms enabled can affect this? Here are the ones enabled in React Native - https://github.com/facebook/react-native/blob/master/babel-preset/configs/main.js

[gaearon]

Hard to say. My guess would rather be that core-js 1.x and 2.x WeakMaps are incompatible but it’s, too, a shot in the dark. Maybe @zloirock has insights.

[zloirock]

@zloyrock

@zloirock :)

core-js 1.x and 2.x WeakMaps are incompatible

They are compatible.

Is it possible that the Babel transforms enabled can affect this?

It's possible - babel runtime transformer should not transform core-js. Most likely it's this or a similar problem.

Without a reproducible example, I have no any other ideas.

[gaearon]

Thanks for chiming in 👍

I’m trying to come up with reproducible example but there are many variables that come into play (e.g. hot reloading was recently added to RN which will import that WeakMap as part of initialization code) so it’s a bit hard to isolate the offending behavior.