Breaking Changes:

• We will stop publishing new versions of flow-parser-bin. If you want to parse flow code with native-like performance, you should use hermes-parser package, which is running on wasm.

Likely to cause new Flow errors:

• The deprecated React$Element type is now removed. You should use React.Node or React.MixedElement in most cases. If you really want exact react elements, you can use ExactReactElement_DEPRECATED

New Features:

• Flow has full React 19 ref-as-prop support. Since this version, you are allowed to spread ref prop in component syntax components.

IDE:

• We now provide a rewrite refactor to convert arrays, map, set, and object types into their readonly forms.

Breaking Changes:

• Starting from this version, flow-bin will only contain ARM64 binaries for macOS.

Likely to cause new Flow errors:

• Flow now errors on constant conditions and invalid comparisons. example
• Flow will no longer infer an unsound type for array and object literals passed to a general function. example
• React.ElementConfig<...> and React.PropsOf<...> will now always include the ref prop in the props object type.
• The config option for an earlier mode react.ref_as_prop=partial_support is removed, and Flow now has better support for React 19's ref-as-prop behavior:
  1. All the ref prop in component syntax components must be made optional
  2. You might need to rewrite some of your higher order components. e.g. previously an identity HOC must be written as

declare function id<Props: {...}, Ref>(c: component(ref?: Ref, ...Props)): component(ref?: Ref...Props)

now it must be written as

declare function id<Props: {...}>(c: component(...Props)): component(...Props)

Library Definitions:

• Fix type of useActionState hook in React. Type should be Awaited instead of Promise (the opposite operation).
• Added types for Object.groupBy and Map.groupBy
  • https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/groupBy
  • https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/groupBy

Bug fixes:

• Fixed a bug that when experimental.ts_utility_syntax=true, imported Readonly, ReadonlyArray, NonNullable, ReadonlyMap, ReadonlySet types are considered any.

Likely to cause new Flow errors:

• Flow now strictly enforces that Array.includes, .indexOf, and .lastIndexOf take only values that could potentially be present in the array -- it is now an error to check if arrays include values whose types are unrelated to the array's element type. This catches many cases where developers accidentally test the wrong thing, which may lead to errors at runtime.
• Flow will infer general types for react jsx elements. The new behavior is described in the docs.

Likely to cause new Flow errors:

• Invalid suppressions like // $FlowFixMe[incompatible will now cause an error on the suppression

Likely to cause new Flow errors:

• Multiple object property invariant subtyping errors on the same object will be combined into one error. Some error locations might be moved.

Notable bug fixes:

• Fixes crash happening on exported functions with signatures of the form <X>(x: X, y: typeof x): T

IDE:

• We now provide a best effort quickfix for some invariant subtyping errors.

Library Definitions:

• Array.find can now return a more precise value if the predicate is inferred to be a type guard. example

Breaking Changes:

• Support for the casting_syntax=colon is removed. This means that the as casting syntax is always enabled.
• experimental.error_code_migration config option is removed. In the previously version, the only allowed value was new.

Likely to cause new Flow errors:

• We have fixed more source of object literal unsoundness. More errors will be exposed. (example)
• For most of the common invariant subtyping errors (arrays, objects with mutable fields), we will now emit a single subtyping error with explanation how to fix. Error locations might change. See https://flow.org/en/docs/lang/variance/#toc-invariance to learn more about variance of types
• Suppressions without error code will now be an error instead of a warning.
• Suppressions without error code will no longer apply. You can download and use this script to automatically migrate most of the suppressions.
• Only $FlowFixMe and $FlowExpectedError are supported suppression comments. Existing $FlowIgnore and $FlowIssue will no longer suppress anything.

New Features:

• Added a Flow lint require-explicit-enum-checks which works like require-explicit-enum-switch-cases but for match.

Notable bug fixes:

• Various codemods will now insert parentheses around expressions in class extends if they are changed. Previously, it might result in invalid code like class Foo extends myExpr() as Bar
• Fixes an inconsistency in recording the this-type guard on a regular (non-declare) method in the same vs different files.

IDE:

• Added a quickfix to convert type to value imports on type-as-value errors.

Likely to cause new Flow errors:

• Accessing missing exports on namespaced import will now trigger missing-export error instead of prop-missing error. (example)
• The only supported suppress_type $FlowFixMe is now just a type alias of any. For most of the code, there will be no functional differences. However, you might see new errors if you have any local definitions of $FlowFixMe, or you used the undocumented $FlowFixMe<arbitrary type arguments>.
• Support for suppress_type config has been removed. The only supported supress_type now is $FlowFixMe. If you want other variants, you can add

type MySuppressType = any

in your global library definitions.

• Many subtyping type errors' error codes have been standardized into incompatible-type, so some previously suppressed errors will reappear until you change the suppression error code into incompatible-type. The change was announced in the previous version, with option to enable it via experimental.error_code_migration=new. Now the only valid option to experimental.error_code_migration is new. You can run flow codemod error-code-migration --write . with the previous version of Flow to help migrate, since the codemod is removed in this version.

New Features:

• In the next release (0.280.0) of Flow, we intend to standardize the error code for various subtyping errors into incompatible-type. You can add experimental.error_code_migration=new in your flowconfig to enable the new behavior now. We also provide a codemod flow codemod error-code-migration --write . that you can run over your codebase to automatically change the error code. Both the flowconfig option and the codemod will be removed in the next version.

Notable bug fixes:

• Improved precision of error messages when inferred primitive types are checked against other incompatible primitive types (e.g. try-Flow)

Misc:

• Thanks @JamBalaya56562 for fixing various typos across the codebase!

Likely to cause new Flow errors:

• Hooks calls inside normal functions in component or hooks as conditional calls. They will get react-rule-hook-conditional error instead of react-rule-hook error.
• Array literals that cannot be contextually typed will be inferred as an actual array type. It might cause additional errors. example