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.