I would like to suggest the following:

• github.com/golang/go
• github.com/twbs/bootstrap

Try following the React.js project itself on GitHub. There are some very good minds working on it. I am sure you'll learn a lot.

github.com/facebook/react

Like Jill Castro has said, the React project itself is exemplary. However, If you're looking for projects that are built with React, I can highly recommend this list.

Just by scrolling through it you'll notice a lot of big-name companies. A lot of them have their code open-sourced. I personally really like the Khan Academy repository.

Any project written in TypeScript will be because there is a compiler checking for consistency. Smooth refactoring and maintainability are part of the value proposition of TypeScript.

A notable React project written in TypeScript is Microsoft's TypeScript-React-Starter.