"unused" option in uglifyjs webpack prod config behaves unexpectedly

[jslegers]

Is this a bug report?

Yes

Steps to Reproduce

1. Install app
2. run dev server
3. build app
4. run build

Expected Behavior

Both the dev code & the built code should behave the same.

Actual Behavior

Some of the built code behaved incorrectly.

More specifically, some WebGL code generated with Emscripten did not behave as expected in the built code.

In my app, I expected to see this :

Instead, I saw this :

Setting the "unused" flag to "false" fixes this problem.

Demo

A demo app can be found at https://github.com/jslegers/luciadria-react-app.

Unfortunately, this demo requires LuciadRIA as a local dependency, and unfortunately I'm not allowed to share this code as it is proprietary. However, it should give an idea of what I'm trying to achieve.

Considering the behavior of the "unused" flag is unreliable, it makes sense to set it's default value to "false", like the "comparisons" flag.

[gaearon]

Would it be more appropriate to file this with Uglify?

[jslegers]

@Gaeron :

I can't really tell which of the underlying components is responsible. Is it webpack-contrib/uglifyjs-webpack-plugin? Or maybe mishoo/UglifyJS2? I'm not sure how to tell.

I can, however, tell that setting the unused flag in the webpack.config.prod.js config file of facebook/create-react-app fixes the issue.

And considering setting the unused flag to true can actually break an app and this behavior is unpredictable, it seems reasonable to me to set the default to false for a boilerplate project like facebook/create-react-app, as is already the case for the comparisons flag (see #2376).

You don't agree?

[stale]

This issue has been automatically marked as stale because it has not had any recent activity. It will be closed in 7 days if no further activity occurs.