To increase the quality of the codebase and contributions, we are considering to move from plain JavaScript to TypeScript.
For more details, please see https://github.com/strapi/strapi/issues/630.
Since 16/03/2022, TypeScript is available in beta more details on how to test it here: https://forum.strapi.io/t/strapi-v4-2-0-beta-with-typescript-support-dark-mode/16732