Flow

Flow is a static type checker for JavaScript. It was designed to help improve code quality and developer productivity. It does this through several smart capabilities. First, it identifies problems as you code, so you no longer have to waste time guessing and checking again and again. Second, it understands your code and makes its knowledge available, allowing you to build other smart tools on top of it. Third, it helps you refactor safely so you can focus on the changes you want to make and not on what you might break. Lastly, it can help prevent bad rebases and protect your carefully designed library, which is especially relevant when working with a large group of developers. Flow integrates with many tools, so you can easily and seamlessly insert it into your existing workflow and toolchain.