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Build a library

Publish ESM and CommonJS with dependencies left external.

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A library is the opposite of an app: the code you import is not yours to bundle, and the consumer's module system decides the format.

guchho build src/index.js --format=esm --outdir=dist/esm \
  --external:react --external:react-dom
guchho build src/index.js --format=cjs --outdir=dist/cjs \
  --external:react --external:react-dom

--external: takes a value after a colon rather than an equals sign because it may be repeated: one flag per package you do not want to bundle.

Match the consumer

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  • --format=esm for bundlers and modern import.
  • --format=cjs for require and older Node.
  • --platform=neutral when the library runs both in browsers and in Node and must not assume either's globals.
  • --target=es2020 (or similar) to keep from emitting syntax an old runtime cannot parse - see targets and browsers.

Keep the output tree-shakeable

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Guchho does not tree-shake unless asked. For a library that is imported by name, turn it on and let the consumer keep only what it uses:

guchho build src/index.js --format=esm --bundle --tree-shaking --outdir=dist

See tree shaking for the rules that make an export removable.

Declare the entry points

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Point package.json at what you built so tools find it:

{
  "exports": {
    ".": {
      "import": "./dist/esm/index.js",
      "require": "./dist/cjs/index.js"
    }
  }
}

See also

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