Lexical variables in the outermost lexical scope don’t get mangled

Lexical variables in the outermost lexical scope don’t get mangled, i.e. they appear to be incorrectly treated as global variables.

To Reproduce

Minimal code to reproduce the bug:

// foo.mjs
const foo = 42;
const bar = 64;

$ minify foo.mjs

Actual Output

const foo=42,bar=64;

Expected Output

const a=42,b=64;

I’m using the babel-minify CLI, v0.4.3, with no additional configuration.

As a workaround, I have to wrap my source code in a block: { … }. That way, babel-plugin-minify-mangle-names seems to kick in correctly.

[mathiasbynens]

To clarify what I mean:

1. Enabling --mangle.topLevel for .mjs files (as @boopathi suggested) solves the problem for modules. It would be great to do this!
2. Adding something like --mangle.topLevelLexical would enable this functionality for non-modules too.

[bakkot]

@mathiasbynens, for what it’s worth, I would be very surprised if those names were changed without --mangle.topLevel. I expect the default behavior to be that compiled programs have the same observable semantics as the input, modulo Function.prototype.toString and function names (and also stack traces and other non-spec things). And the names in the top-level lexical scope are very much part of the observable semantics of a program, just like names in the global scope.