The composite option enforces certain constraints which make it possible for build tools (including TypeScript itself, under --build mode) to quickly determine if a project has been built yet.

When this setting is on:

• The rootDir setting, if not explicitly set, defaults to the directory containing the tsconfig.json file.

• All implementation files must be matched by an include pattern or listed in the files array. If this constraint is violated, tsc will inform you which files weren't specified.

• declaration defaults to true

You can find documentation on TypeScript projects in the handbook.