In addition to using ES6 as a specific flavor, on top of JavaScript, as a technology, we're using TypeScript for it's additional type safety and resulting confidence when developing long lasting, well-rounded frontend solutions like a component library.

One additional feature of TypeScript we're relying heavily on is the ability to generate types for publishing packages. We use this to generate all component typings automatically, based on shared JSON Schema definitions, enabling auto-complete for properties defined in the context of the component... based on the generated types.