how to use existing react app in typescript code example

Example 1: add typescript in create react app

npm install --save typescript @types/node @types/react @types/react-dom @types/jest
# or
yarn add typescript @types/node @types/react @types/react-dom @types/jest

Example 2: create new react app using typescript

create-react-app app-name --scripts-version=react-scripts-ts