flow

Flow is a tool that checks your JavaScript code for type errors before you run it. Instead of discovering bugs at runtime, Flow lets you catch mistakes like passing a number where a string is expected — immediately, as you write code. This saves time debugging and makes refactoring safer because you know right away if you've broken something.

Here's how it works: you add lightweight type annotations to your JavaScript code (like marking a function parameter as a string or a number). Flow then reads your code and verifies that you're using values correctly throughout your program. If there's a mismatch — say you're trying to call a text function on a number — Flow tells you about it before the code runs. You don't have to change how JavaScript works; Flow just watches over it like a careful reviewer.

The tool is designed for JavaScript developers who want more confidence in their code without switching to a completely different language. If you're working on a medium or large codebase, Flow helps prevent entire categories of bugs that would normally only show up when users encounter them. Teams use it to speed up development cycles — less time firefighting production issues, more time shipping features.

Flow itself is written in OCaml, a programming language good at analyzing and transforming code, which is why it can understand JavaScript's structure so deeply. The README notes that you'll need to install it as a command-line tool on your computer (not through npm like most JavaScript packages), but once it's on your path, running Flow on your project is straightforward. If you're curious to learn more, the documentation is at flowtype.org.