V2 Scope hoisting seems to break renaming

[jfrconley]

🐛 bug report

Enabling scope hoisting seems to break variable renaming resulting in a significantly increased bundle size.

This may be related to #3664, but I have verified that I have @babel/travers at 7.10.1

🎛 Configuration (.babelrc, package.json, cli command)

parcel build --target lambda --no-source-maps dist/index.js --detailed-report

{
  "lambda": "bundle/index.js",
  "targets": {
    "lambda": {
      "context": "node",
      "sourceMap": false,
      "includeNodeModules": {
        "@acuris/aws-es-connection": false,
        "@elastic/elasticsearch": false,
        "sharp": false,
        "aws-sdk": false,
        "jsonata": false,
        "pino-pretty": false,
        "jmespath": false
      }
    }
  }
}

🤔 Expected Behavior

Enabling scope hoisting should have shortest possible top level variable names, reducing overall bundle size.

😯 Current Behavior

While scope hoisting itself seems to be working, top level values all have super long generated names (ex: $b10acc16e6462fee040168c6ed2f0$var$actualClose), significantly increasing bundle size. In my case, the scope hoisted size was double that of simple bundling.

🌍 Your Environment

Software	Version(s)
Parcel	2.0.0-nightly.277
Node	12
npm/Yarn	1.22.4
Operating System	Ubuntu 20.04

[mischnic]

hile scope hoisting itself seems to be working, top level values all have super long generated names (ex: $b10acc16e6462fee040168c6ed2f0$var$actualClose), significantly increasing bundle size. In my case, the scope hoisted size was double that of simple bundling.

I'm fairly confident that you have an eval somewhere in the bundle, which deopts Terser.

[jfrconley]

Just confirmed that there are no calls to eval or use of the Function constructor anywhere in my bundle.

[mischnic]

Strange, even if you build with --no-minify?

[jfrconley]

Yes, just double checked. The minify option seems to only apply to contents of modules, not the bundle

[mischnic]

The minify option seems to only apply to contents of modules, not the bundle

Parcel 1 ran Terser per file/asset, but Parcel 2 does run it on the concatenated bundle

[jfrconley]

Is there a documented list of things that cause terser to bail out? I'm testing this on a very small bundle (13 KB) and can't find anything that seems likely.

EDIT: Tried running terser directly on output, and it did nothing. Seems like it's bailing out for some reason.

[jfrconley]

I figured it out. Terser does not mangle top level names by default, but requires the toplevel option to be set. Parcel should probably do this by default when scope hoisting.

https://terser.org/docs/cli-usage#cli-mangle-options

[mischnic]

We should:

parcel/packages/optimizers/terser/src/TerserOptimizer.js

Lines 31 to 33 in 2668bee

	toplevel:
	bundle.env.outputFormat === 'esmodule' ||
	bundle.env.outputFormat === 'commonjs',

Your outputformat should be commonjs, right?

[jfrconley]

That only applies to compression, not mangling. The issue goes away when I have .terserrc with the following config.

{
	"mangle": {
		"toplevel": true
	}
}

[mischnic]

👍 Thanks for investigating!

[fregante]

It seems that the suggested way to handle this issue is to enable variable name mangling through minification, which makes variables less readable rather than more readable.

Can Parcel do what Rollup does to generate readable code? My code has been rejected by a store due to “obfuscation”

[mischnic]

@fregante That's very unfortunate, can you open a new issue for that? Maybe we can try to shorten the names

[fregante]

Thanks! Done