Limits of CoffeeScript `--bare` flag for reducing bundle size in low‑traffic workloads

Answer

Compiling a minimal CoffeeScript file with the --bare flag removes the implicit IIFE wrapper that the compiler normally adds around each file. This saves roughly the wrapper’s overhead – about 20‑30 bytes of raw JavaScript per file. After minification and gzip, the transferred‑byte reduction is typically a fraction of a byte per module, so for low‑traffic workloads the saving is only noticeable when many small modules are compiled separately.

Confirmed facts

• The wrapper is a simple (function(){ ... }).call(this); block; removing it reduces the source size by a constant amount per file.
• Class definitions and splat arguments generated by CoffeeScript do not rely on the wrapper’s scope for correct execution; they contain all necessary bindings within the generated function.
• Top‑level variables that are intended to be file‑private become global when the wrapper is omitted, which can change runtime behavior if those variables are accessed elsewhere.
• The CoffeeScript compiler still produces correct source maps for --bare output; the maps reflect the line offsets of the unwrapped code.

Likely explanation

In a bundler such as Webpack or Rollup, each module is already wrapped by the bundler’s own module format (e.g., IIFE or ES‑module wrapper). Adding the CoffeeScript --bare flag therefore yields little extra size reduction because the bundler’s wrapper already eliminates the CoffeeScript IIFE. The main benefit of --bare appears only when you are compiling CoffeeScript to plain JavaScript files that are served individually (e.g., via a simple static server).

Steps to verify impact for your project

1. Build the project twice: once with the default CoffeeScript compiler and once with --bare. Keep all other build steps identical.
2. Measure the size of the generated bundle (or individual files) before and after gzip, e.g.:

   wc -c bundle.js          # raw bytes
   gzip -c bundle.js | wc -c  # gzipped bytes
   

3. Run the full test suite after enabling --bare to ensure no runtime errors appear from missing wrapper scoping (especially check for accidental globals).
4. Inspect the generated source map (e.g., open the map in devtools) to confirm that line numbers align with the original CoffeeScript sources; no extra adjustment is needed if you use the map directly with the bare output.

When to ask for more detail

If you are unsure whether your workload consists of many small, separately served modules or a single bundled file, please clarify the number of CoffeeScript modules you compile. That detail changes the recommendation: many small modules may justify --bare for measurable savings; a single bundle typically does not.