Elm minification is not two-phase

[Janiczek]

🐛 bug report

To be most effective, Elm minification with uglifyjs should be done in two phases. It is currently done in one phase.

🤔 Expected Behavior

The minification process does something analogous to this:

uglifyjs elm.js --compress 'pure_funcs="F2,F3,F4,F5,F6,F7,F8,F9,A2,A3,A4,A5,A6,A7,A8,A9",pure_getters,keep_fargs=false,unsafe_comps,unsafe' --output=elm.phase1.js
uglifyjs elm.phase1.js --mangle --output=elm.min.js # phase 2

As seen in the official Elm page:

Note 1: uglifyjs is called twice there. First to --compress and second to --mangle. This is necessary! Otherwise uglifyjs will ignore our pure_funcs flag.

😯 Current Behavior

Currently the options passed to uglifyjs have both compress and mangle options set, and uglifyjs is only called once, as seen here: https://github.com/parcel-bundler/parcel/blob/master/src/assets/ElmAsset.js#L89-L119

Thus, there are gains to be ... gained! 🤔 😅

💁 Possible Solution

Let's call the uglifyjs twice, with two different options, closely modelling the recommended CLI usage.

[kzc]

To be most effective, Elm minification with uglifyjs should be done in two phases. It is currently done in one phase.

That's not required in Terser because pure_funcs works correctly with both compress and mangle enabled in the same minify invocation.

pure_funcs in uglify-js can also be made to work correctly with both compress and mangle enabled if the rename pass is disabled via --no-rename.

See:

• Add support for Elm assets #1968 (comment)
• Use uglifyjs --no-rename flag to compress and mangle in one step with pure_funcs rtfeldman/elm-spa-example#54

However, the caveat is that parcel only performs Terser compress per JS asset and does not perform compress on the final bundle. It would produce a smaller bundle if each asset were not minified individually, but instead only invoke minify on the final bundle - as webpack and rollup typically do.

[Janiczek]

Ah, thanks for clarifying!