Function resolveOutputJsExtension

packages/vite/src/node/build.ts:985–994

(
  format: ModuleFormat,
  type: string = 'commonjs',
)

type JsExt = 'js' | 'cjs' | 'mjs'

function resolveOutputJsExtension(
  format: ModuleFormat,
  type: string = 'commonjs',
): JsExt {
  if (type === 'module') {
    return format === 'cjs' || format === 'umd' ? 'cjs' : 'js'
  } else {
    return format === 'es' ? 'mjs' : 'js'
  }
}

export function resolveLibFilename(
  libOptions: LibraryOptions,