When no --format flag is specified..

tsup src/index.ts

..tsup can look in package.json for clues.

• The main, module, types/typings, and unpkg/jsdelivr fields all imply the preferred output paths, even allowing for custom bundle names.
• When module exists, tsup creates an ESM bundle.
• When types or typings exist, tsup creates a .d.ts bundle.
• When unpkg or jsdelivr exist, tsup creates a UMD bundle.