Flow now decides the type of string, number, and boolean literal values at their definition site: either the broad type (e.g. string) or the literal type (e.g. 'foo'). You can control the behavior with type annotations, as const, or const type parameters.

Mapped types let you derive one object type from another by transforming each of its properties, and power new utility types like Pick, Omit, and Record.

Flow is excited to announce Component Syntax, adding first-class support for React primitives such as components and hooks to the Flow language. These features bring improved ergonomics, expressiveness, and static enforcement for many of the Rules of React.

Conditional types allow you to choose between two different output types by inspecting an input type.

Flow now lets you define a function that encodes a type predicate over its parameter.

To make it easier for new users to get started with Flow, we’re updating our type casting syntax to use 'as'.

Labeled tuple elements, read-only tuples, optional tuple elements, tuple spread, and more.

As of v0.201.0, Flow can now lint against unused/floating Promises. Unused promises can be dangerous, because errors are potentially unhandled, and the code may not execute in the intended order. They are usually mistakes that Flow is perfectly positioned to warn you about.

Starting in Flow version 0.201, make an object type’s fields all optional using Partial<ObjType> (use instead of the unsafe $Shape), and make an object type’s optional fields required with Required<ObjType>.

We announced 5 years ago a plan to eventually make exact object types the default. We are now proceeding with this plan.

Local Type Inference makes Flow’s inference behavior more reliable and predictable, by modestly increasing Flow’s annotation requirement, bringing it closer to industry standard and capitalizing on increasingly strongly and explicitly typed codebases.

Flow handled the empty object literal in a permissive but unsafe way. The fix described in this post increases safety and predictability, but requires using different patterns and behavior.

Flow will now require more annotations to functions and classes.

Flow is releasing a new language rule that determines the type of an unannotated variable at its initialization. Along with these new rules come several fixes to soundness bugs that were causing refinements to not be invalidated.

We're replacing Flow’s current inference engine with a system that behaves more predictably and can be reasoned about more locally.

Flow Enums are an opt-in feature which allow you to define a fixed set of constants which create their own type.

A comparison of the enums features of TypeScript and Flow.

Flow’s Indexed Access Types are a new type annotation syntax that allows you to get the type of a property from an object, array, or tuple type.

Improvements in soundness for this-typing in Flow, including the ability to annotate this on functions and methods.