Flow.org

Flow is a static type checker for JavaScript that we built at Facebook. The overall mission of Flow is to deliver an immersive coding experience for JavaScript developers—a fast edit-refresh cycle—even as the codebase evolves and grows. In engineering terms, we identify two concrete goals that are important to this mission: precision and speed. These goals pervasively drive the design and implementation.

Precision

JavaScript bugs can have significant impact at Facebook. Developers want to find and fix as many bugs as they can by the time their code rolls out into production. So we must care about soundness. At the same time, we must also care about not reporting too many spurious errors, because a low signal/noise ratio implies a low fix rate.

In other words, we want Flow’s analysis to be precise in practice—it must model essential characteristics of the language accurately enough to understand the difference between idiomatic code and unintentional mistakes.

Speed

Precision usually comes at the cost of speed. But while a precise analysis is desirable, most of the appeal of JavaScript is lost if we slow down the edit-refresh cycle by making developers wait as we compile.

In other words, we must engineer Flow’s analysis to be extremely fast—it must respond to code changes without noticeable delay, while still being precise enough in practice.