We use TypeScript whenever possible. When using Webpack to bundle our code, we'll pass TS files through ts-loader.
We use a combination of ESLint and Prettier to enforce a consistent code style. We make use of IDE extensions to automatically lint and format our code.
If the project is not TypeScript-based, we usually use the Airbnb config. If it is TypeScript, we use the recommended settings that come with typescript-eslint.