This post announced component and hook syntax for React when it was introduced. For current syntax and behavior, see the Component Syntax and Hook Syntax documentation.
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.
]]>This post announced conditional types when it was introduced. For current syntax and behavior, see the Conditional Types documentation.
Conditional types allow you to choose between two different output types by inspecting an input type.
]]>This post announced user-defined type guards when it was introduced. For current syntax and behavior, see the Type Guards documentation.
Flow now lets you define a function that encodes a type predicate over its parameter.
]]>This post announced the as type casting syntax when it was introduced. For current syntax and behavior, see the Type Casting documentation.
To make it easier for new users to get started with Flow, we’re updating our type casting syntax to use 'as'.
]]>This post announced new tuple type features when it was introduced. For current syntax and behavior, see the Tuples documentation.
Labeled tuple elements, read-only tuples, optional tuple elements, tuple spread, and more.
]]>This post announced unused Promise detection when it was introduced. For current syntax and behavior, see the Linting documentation.
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.
]]>This post announced Partial, Required, and catch annotations when it was introduced. For current syntax and behavior, see the Utility Types documentation.
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>.
This post announced exact object types becoming the default when it was introduced. For current syntax and behavior, see the Objects documentation.
We announced 5 years ago a plan to eventually make exact object types the default. We are now proceeding with this plan.
]]>This post announced local type inference when it was introduced. For current syntax and behavior, see the Annotation Requirement documentation.
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.
]]>This post announced improved handling of the empty object when it was introduced. For current syntax and behavior, see the Objects documentation.
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.
]]>This post announced increased annotation requirements for functions and classes when it was introduced. For current syntax and behavior, see the Annotation Requirement documentation.
Flow will now require more annotations to functions and classes.
]]>This post announced the constrained writes rule when it was introduced. For current syntax and behavior, see the Variables documentation.
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.
]]>This post announced local type inference when it was introduced. For current syntax and behavior, see the Annotation Requirement documentation.
We're replacing Flow’s current inference engine with a system that behaves more predictably and can be reasoned about more locally.
]]>This post announced Flow Enums when it was introduced. For current syntax and behavior, see the Flow Enums documentation.
Flow Enums are an opt-in feature which allow you to define a fixed set of constants which create their own type.
]]>This post announced TypeScript Enums vs. Flow Enums when it was introduced. For current syntax and behavior, see the Flow Enums documentation.
A comparison of the enums features of TypeScript and Flow.
]]>This post announced indexed access types when it was introduced. For current syntax and behavior, see the Indexed Access Types documentation.
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.
]]>This post announced sound typing for this when it was introduced. For current syntax and behavior, see the Functions documentation.
Improvements in soundness for this-typing in Flow, including the ability to annotate this on functions and methods.
This post described Flow's direction and open source engagement at the time of writing and may not reflect the current state of the project.
An update on Flow's direction and open source engagement.
]]>This post announced Types-First becoming the only supported mode when it was introduced. Syntax and behavior may have changed since this post was written.
Types-First will become the only mode in Flow in v0.143 (mid Jan 2021).
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