Microbundle

{
  "name": "foo",                      // your package name
  "type": "module",
  "source": "src/foo.js",             // your source code
  "exports": {
    "require": "./dist/foo.cjs",      // used for require() in Node 12+
    "default": "./dist/foo.modern.js" // where to generate the modern bundle (see below)
  },
  "main": "./dist/foo.cjs",           // where to generate the CommonJS bundle
  "module": "./dist/foo.module.js",   // where to generate the ESM bundle
  "unpkg": "./dist/foo.umd.js",       // where to generate the UMD bundle (also aliased as "umd:main")
  "scripts": {
    "build": "microbundle",           // compiles "source" to "main"/"module"/"unpkg"
    "dev": "microbundle watch"        // re-build when source files change
  }
}