remove console calls on production

[Emilios1995]

This adds to the production mode of babel-preset-react-native, a plugin to delete calls to the console while still preserving the side effects inside them.
The motivation is the great improvement on performance on the code running on device after removing the calls to console methods

Note that although the plugin works as expected, I don't know if this could cause any inconveniences on the rest of the system.

[ghost]

By analyzing the blame information on this pull request, we identified @skevy and @frantic to be potential reviewers.

[Emilios1995]

CC @javache@bestander

[Emilios1995]

Notice: The commit was amended to simplify my original transform function.

[bestander]

Thanks, @Emilios1995, I'll post this internally at FB to ask teams if this affects them in any way and get back to you.
Don't be shy to nudge if I get distracted and forget to get back, but I'll try not to

[javache]

We should keep console.error since that provides non-fatal error logging. You should also verify this doesn't break our console polyfill code and console error reporter code.

[javache]

This is identical to https://github.com/babel/babel/tree/master/packages/babel-plugin-transform-remove-console

[Emilios1995]

@javache I've changed the transform to ignore console.error
It's not identical, since mine preserves the side effects, as @ide suggested in #8285

[ide]

Instead of creating a new function object each time, it might be simpler just to use Function.prototype, which is a no-op function #jstrivia

[Emilios1995]

So, @bestander, do you think it's safe to merge?

[bestander]

@Emilios1995, it seems that we use console.log in some products internally even for prod builds.
Any ideas how not to do it by default?
Can this plugin be in userland instead of baked into plugin?

[Emilios1995]

Well, we could just suggest in the docs to include the official transform-remove-console in the production environment of the user's own .babelrc

[bestander]

Yes, @Emilios1995, I think it is a good idea, want to send a docs PR?
Adding this setting by default may be confusing to people.

[bestander]

Let's close this for now, feel free to reopen if there are more ideas

[stovmascript]

I tried using babel-plugin-transform-remove-console as specified in the performance docs in my RN project (v0.34.1) and also in a newly initialized AwesomeProject (v0.35.0), but it looks like the RN packager doesn't like the transformed code. Both throw the same error when running assembleRelease with gradle:

:app:bundleReleaseJsAndAssets
[2016-10-14 14:51:37] <START> Building Dependency Graph
[2016-10-14 14:51:37] <START> Crawling File System
[2016-10-14 14:51:37] <START> Finding dependencies
[2016-10-14 14:51:41] <END> Crawling File System (3999ms)
[2016-10-14 14:51:41] <START> Building in-memory fs for JavaScript
[2016-10-14 14:51:41] <END> Building in-memory fs for JavaScript (218ms)
[2016-10-14 14:51:41] <START> Building in-memory fs for Assets
[2016-10-14 14:51:41] <END> Building in-memory fs for Assets (165ms)
[2016-10-14 14:51:41] <START> Building Haste Map
[2016-10-14 14:51:41] <START> Building (deprecated) Asset Map
[2016-10-14 14:51:41] <END> Building (deprecated) Asset Map (59ms)
[2016-10-14 14:51:42] <END> Building Haste Map (298ms)
[2016-10-14 14:51:42] <END> Building Dependency Graph (4691ms)
TransformError: /Users/foo/AwesomeProject/node_modules/react-native/Libraries/JavaScriptAppEngine/Initialization/ExceptionsManager.js: /Users/foo/AwesomeProject/node_modules/react-native/Libraries/JavaScriptAppEngine/Initialization/ExceptionsManager.js: Property right of AssignmentExpression expected node to be of a type ["Expression"] but instead got null
:app:bundleReleaseJsAndAssets FAILED
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':app:bundleReleaseJsAndAssets'.
> Process 'command 'node'' finished with non-zero exit value 1

I also tested it on another project for the web, which is bundled with webpack and works as expected, so I didn't open an issue at the babel repo yet because it might be RN related.