Typing React
Prior to the dramatic rise in the popularity of TypeScript, React programmers either went without any sort of type system or used libraries such as Flow or PropTypes.
Flow is another library developed by Facebook with the intent of adding types to JavaScript. It has similar goals to TypeScript but takes a different route to achieve them. Instead of being a superset of JavaScript, Flow uses comments and type annotations checked by a language server, which are then removed by a transpiler such as Babel. Since both libraries were developed by Facebook, it was common to use them together, but the popularity of Flow has waned as TypeScript has emerged as the type system of choice for web developers.
PropTypes is another library for enforcing type-checking. In this case, the library is specifically for use with React and has the narrow focus of checking types on React "props," or the parameters that are passed along with components.