Core-js: Test error after upgrading to corejs 3

[mohsinulhaq]

After using {corejs: 3} option for both @babel/preset-env and @babel/plugin-transform-runtime, and using @babel/runtime-corejs3, my tests are failing with the following error:

/node_modules/core-js-pure/internals/well-known-symbol.js:10
  return store[name] || (store[name] = NATIVE_SYMBOL && Symbol[name]
                                                              ^
TypeError: Cannot read property 'iterator' of undefined

[zloirock]

After using {corejs: 3} option for both @babel/preset-env and @babel/plugin-transform-runtime...

You should you corejs: 3 in @babel/preset-env OR in @babel/plugin-transform-runtime, but not together.

[mohsinulhaq]

I am using the useBuiltins: 'usage' option for @babel/preset-env, so I get this warning if I remove corejs: 3 from options:

WARNING: We noticed you're using the `useBuiltIns` option without declaring a core-js version. 
Currently, we assume version 2.x when no version is passed.
Since this default version will likely change in future versions of Babel,
we recommend explicitly setting the core-js version you are using via the `corejs` option.

[zloirock]

Ok, let say in other words: you should use useBuiltIns option of preset-env OR corejs option of runtime, but not together.

[mohsinulhaq]

https://github.com/babel/babel/issues/10121#issuecomment-505002153
yeah I just found that out :)

But even after this, I get the exact same error.

[zloirock]

I hope that you don't transform core-js in your node_modules?

[mohsinulhaq]

No, I don't think so.

[mohsinulhaq]

Scrolling down, I see another error:

/node_modules/@babel/core/lib/config/files/configuration.js:206
    throw err;
    ^

TypeError: /.babelrc.js: Error while loading config - Cannot read property 'species' of undefined
    at module.exports (/node_modules/core-js-pure/internals/well-known-symbol.js:10:63)

[zloirock]

Seems it's a similar error.

Take a look at well-known-symbol.js code. NATIVE_SYMBOL && Symbol[name] will be executed only when NATIVE_SYMBOL is true. NATIVE_SYMBOL will be true only when Symbol is a function. So this error is possible only if this code is changed (most likely by transpiler) or someone directly removed global Symbol before some parts of core-js will be executed.

[mohsinulhaq]

I will check what is causing this, but I have a question. The environment where these tests are being run is Node 10.14.1, where Symbol is natively supported. Why is Symbol not available here? Why am I encountering the polyfill of Symbol?

[zloirock]

It should not be polyfilled, just wrapped in you uses a version without global pollution.

[mohsinulhaq]

Yeah, so when it calls Symbol[name], the Symbol should be the one in the node environment, right? In which case can this turn up undefined?

[zloirock]

See this comment, this is exactly the answer to your question.

[mohsinulhaq]

Hi, just putting it there if it helps:

While debugging, I found that Symbol from closure is undefined at this time, but global.Symbol is not. Does that tell you anything?

[mohsinulhaq]

When I replace Symbol with global.Symbol, and then apply breakpoint, I find that global.Symbol is undefined, while Symbol from closure is not. 🤷‍♂

[mohsinulhaq]

In another case where I try to use Symbol || global.Symbol, both are undefined

[zloirock]

Looks like core-js code is transpiled. Could you check it in your configs, in the bundle?

[mohsinulhaq]

That's the thing, my build is working fine.
But when I run mocha test, which I do by loading @babel/register before running tests, I get this error. As such I don't have any bundle to check.

[mohsinulhaq]

It seems that the error was due to me doing window.Math = Math in the mocha entry file to work around this issue in jsdom with RxJS:

https://github.com/ReactiveX/rxjs/issues/2651#issuecomment-338648013

Removing this line fixed the issue, however I wonder how could it wipe out global Symbol? 🤔

[zloirock]

Interesting. So, seems you use mocked window and, because of window.Math === Math, it passes current global object detection. But it's not a real global object, so you try to use core-js in a broken environment. You should use core-js before that. Seems the issue resolved.

[mohsinulhaq]

Yes it is, thanks!

[mohsinulhaq]

https://github.com/babel/babel/issues/10133
I have a query regarding corejs and babel. Could you please help me there as well.
Thanks! 😄

[zloirock]

@nicolo-ribaudo already answered to this question.