CoffeeScript default CommonJS module output incompatible with ES‑module bundlers

Question

CoffeeScript default CommonJS module output incompatible with ES‑module bundlers
The CoffeeScript compiler (v1.x) emits CommonJS require and module.exports statements by default. Production builds that bundle the generated JavaScript with tools configured for ES modules (e.g., Webpack, Rollup) often encounter runtime errors such as Uncaught ReferenceError: module is not defined because the bundler expects import / export syntax.
Currently, the compiler offers no flag to change the emitted module system; the --compile and --output options only control file paths. This lack of a module‑format switch is an unresolved design decision: should a --module or --es6 flag be added, or should CoffeeScript rely on a separate transpilation step?
Given this situation, developers need to decide how to integrate CoffeeScript into modern bundling workflows without adding extra build complexity.
Key questions

Should the CoffeeScript project introduce a compiler flag to emit ES module syntax directly?
What are the trade‑offs of adding such a flag versus continuing to rely on post‑processing with Babel or manual edits?
How might future CoffeeScript releases impact existing build scripts that depend on the current CommonJS default?

Tasadduq Burney · Answer

CoffeeScript default CommonJS module output incompatible with ES‑module bundlers
Confirmed facts: The CoffeeScript compiler (v1.x) emits CommonJS syntax (require and module.exports) by default and provides no flag to change the module format. When this output is fed to a bundler that is configured for ES‑module‑only output (e.g., Rollup with output.format = 'es'), the generated globals module and require are undefined, leading to runtime errors such as Uncaught ReferenceError: module is not defined.
Likely explanation
The incompatibility stems from a mismatch between the module system the compiler produces and the one the bundler expects. ES‑module‑only bundlers do not inject a CommonJS compatibility layer unless a plugin is present, so the CommonJS globals remain unresolved.

Compile CoffeeScript to JavaScript: coffee -c src/ -o lib/
If you are using Rollup, add the CommonJS plugin:
   npm i -D rollup-plugin-commonjs
rollup({
  input: 'lib/main.js',
  plugins: [require('rollup-plugin-commonjs')()],
  output: { file: 'bundle.js', format: 'es' }
});
If you are using Webpack, ensure it can process CommonJS (it does by default) or run the compiled output through Babel to convert CommonJS to ES modules:
   npm i -D @babel/core @babel/preset-env
babel lib/ --presets=[@babel/preset-env] --out-dir dist/
Verify the bundle runs without the module is not defined error.

To refine the recommendation, please let me know which bundler you are using (Webpack, Rollup, or another).

CoffeeScript default CommonJS module output incompatible with ES‑module bundlers

CoffeeScript default CommonJS module output incompatible with ES‑module bundlers

Key questions

1 answer

CoffeeScript default CommonJS module output incompatible with ES‑module bundlers

Likely explanation

Your perspective belongs here.

0 question comments