react-native: Packager / server only logs 'Failed building JavaScript bundle' and does not indicate the problematic file

Usually this error is displayed in Expo, but sometimes Expo won’t show it, either because:

• The first build fails so nothing is pushed to Expo
• The previous build failed. Expo shows the error from the previous build, but not the new error!

The easiest solution is to keep your Expo in a good state. I have found Expo running in the iOS Simulator a bit more user friendly than Expo running in the Android Emulator.

For those of you who are stuck in a situation where Expo is not showing the error… I did manage to get the error logged, but it wasn’t pretty!

Starting with jmatsushita’s pointer I made the following changes:

## In node_modules/react-native-scripts/build/util/packager.js
        _log2.default.withTimestamp(_chalk2.default.red('Failed building JavaScript bundle'));
+       _log2.default.withTimestamp(String(err));
## There are TWO lines where you can do this
## But this only showed the error: "Failed to build bundle", so I went to track that down...

## In node_modules/xdl/build/logs/PackagerLogsStream.js
-     bundleError = new Error('Failed to build bundle');
+     bundleError = new Error('Failed to build bundle: ' + JSON.stringify(msg));
## This still didn't contain the error, but I could find where this message originated...

## In node_modules/metro-bundler/src/Server/index.js
## We can include the original error in the message object
      this._reporter.update({
        buildID,
-       type: 'bundle_build_failed' });
+       type: 'bundle_build_failed',
+       error: error });
## There are FOUR places where you can do this!

This got the error displayed on the console. But there must be an easier way… Running a build manually?

Any movement on this? This is most certainly a high priority.

This is making it ridiculously difficult to do any development. I have a syntax error somewhere in my code that I have been hunting for 3 hours.

Unfortunately, @joeytwiddle’s hack doesn’t help me much either…it just produces this:

2:47:22 PM: Failed building JavaScript bundle
2:47:22 PM: Error: Failed to build bundle: {"buildID":"1","type":"bundle_build_failed"}

I’m using expo and deactivating the remote debugger (shaking the phone and clicking stop remote debugging) makes the bundler error message show up on the phone. Better than nothing.

Please delete the labels of this issue, all the info is on the OP post. For the moment, how do you guys go through this? I can’t handle this anymore, my developing time has increased a lot due to lack of info when writing code and receiving an error.

This issue hasn’t been updated in a while. If anyone is still running into this, please go ahead and open a new, separate issue. This will help us debug the situation individually. When a thread gets to this size, it’s hard to keep track when each problem is actually related to the original post.

It seems that this error comes from the react-native-scripts package on this line.