Breaking Changes:

• no_flowlib option is removed. Use builtin_lib=flowlib (default) or builtin_lib=prelude instead.

Likely to cause new Flow errors:

• Render types no longer accept exact react element as arguments. See https://flow.org/en/docs/react/render-types/ for supported cases
• Flow will now always infer a react jsx element to be a render type if it's an element of a component-syntax component, or React.MixedElement otherwise. Legacy code that requires exact react element will see new errors.
• Add an option for rejecting dynamic imports in Haste modules.

New Features:

• Symbol() and Symbol.for() create a unique symbol when they initialize a const, so symbols can be used as property keys without explicit declarations.

Notable bug fixes:

• Bound error type normalization to prevent excessive memory use
• Release temporary signature-hash graphs after Flow merges.
• Using import typeof of a generic value without type arguments keeps its polymorphism instead of reporting missing-type-args, and renders accepts import typeof of a generic component.

Parser:

• Parse deferred imports: import defer * as ns from "m" and import.defer("m") now set the ESTree phase property to "defer".

IDE:

• Hover on members accessed through super shows the declaration
• Hover on destructuring keys shows the member declaration
• Hover on enum members shows the member declaration
• Hover on signature labels and indexer keys shows the parameter
• Hover on component attribute names shows the prop declaration
• Hover on import/export sources shows the module
• Hover on this-parameters shows the parameter
• Hover on indexed-access literals shows the member
• Hover on polymorphic JSX elements no longer exposes overly precise literal types when no annotation demands that precision
• Hover on a call to an overloaded function reports how many other signatures the function has
• Hover lists the signatures of an overloaded declaration instead of printing their intersection
• Component hovers no longer drop indexer rest props
• Align multiline union members in hover type declarations
• Render hover definition links in a compact Rust-style navigation row.
• Expand rest-prop aliases in top-level component hovers.

Library Definitions:

• React ref callbacks now have a stricter return type
• import.meta will no longer have url property and arbitrary unknown property. To add them back, you can do so through declaration merging (e.g. adding interface ImportMeta { url: string } to your global library definitions.
• ArrayLike and ArrayBuffer now follow TypeScript definitions
• Stricter typing for Array(length) and Array(items)

Likely to cause new Flow errors:

• Fixed unsound subtyping of instances against indexer in interfaces (example).
• Flow will no longer emit method-unbinding errors and instead perform stricter this-type checking (example).

IDE:

• Improve the primary type displayed by hover
• Show a single type in type-at-pos and hover
• Truncate oversized hover types and their reference lists

Library Definitions:

• React.ReactElement now matches latest type in @types/react. Added missing React.JSX.Element that makes the type in @types/react .

Likely to cause new Flow errors:

• Flow will now restrict the allowed syntax forms of .call/apply/bind calls. Only expressions of the form <expr>.m.call(<expr>, ...), <expr>.m.apply(<expr>, ...) and <expr>.m.bind(<expr>, ...) are allowed.
• [untyped] section of flowconfig will use the glob system. See https://flow.org/en/docs/config/untyped/

Notable bug fixes:

• Fixed #9407.
• Support instanceof refinement of constructor objects.
• Operations like Partial no longer strip readonly-ness of dictionaries. (example)

Likely to cause new Flow errors:

• More subtyping errors will print the types that are incompatible. Some errors might have new locations.
• A unique symbol annotation is now an error unless it is the type of a const variable, of a readonly object type or interface property, or of a static readonly class field.
• Flow will error on legacy variance syntax in library definition files

New Features:

• We have released flow-parser and flow-eslint packages that are drop-in replacement for hermes-parser and hermes-eslint. Babel docs and ESLint docs have been updated.
• Object properties, class fields, methods, and the members of an interface or declare class imported from another module, can be keyed by a unique symbol.

Notable bug fixes:

• Prevent a race that can leave server hanging
• Support TypeScript constructor objects in Flow class positions.
• keyof over a symbol index signature is now symbol rather than string.
• A class can extend a value with a construct signature.
• x !== undefined no longer stops refining after a call that passes undefined as an argument: (e.g. try-Flow)

Library Definitions:

• Well-known symbols on Symbol are typed as unique symbol, and Symbol.metadata is now available.

Breaking changes:

• [include] and [ignore] sections in flowconfig should use globs instead. Read https://flow.org/en/docs/config/include/ and https://flow.org/en/docs/config/ignore/ for more information.

New Features:

• this type guards can now be used on non-static methods of regular classes, not just declare class and interface methods. The method body is checked for consistency against the guard type.

Notable bug fixes:

• Prevent cached IDE results from blocking rechecks.

Library Definitions:

• Allow symbols to be used as WeakMap keys

New Features:

• In an object type, a bracketed key written as a string or number literal, or as the name of a value, is now a computed key naming a single property rather than an index signature, so several such keys can coexist. A key written as a type-only name is unchanged, and the labeled form [label: K] keeps index-signature semantics. Breaking: a key headed by a value binding is read as a computed key. Thus a class or enum key such as {[C]: V} is now an error, and a qualified type exposed through a class/namespace merge, as in {[C.K]: V}, must use the labeled form {[key: C.K]: V} to remain an index signature.
• In an interface or a declare class body, a bracketed key written as a string or number literal, or as the name of a value, is now a computed key naming a single property rather than an index signature, so several such keys can coexist, and static puts the property on the static side. A key written as a type-only name is unchanged, and the labeled form [label: K] keeps index-signature semantics. Breaking: a key headed by a value binding is read as a computed key, so a class or enum key such as interface I {[C]: V} is now an error and must use [key: C] to remain an index signature.
• A computed method key [expr](): T that resolves to a single literal string or safe-integer number is now accepted in interface, declare class, and object type bodies (previously a parse error outside .d.ts files) and reads as a named method.
• A bracketed access on an array with a string-literal key (for example arr['map'] or arr[Symbol.iterator]) now reads the corresponding named member instead of erroring that the key is not an array index, and a string key holding a safe integer (arr['0']) indexes the array like the equivalent numeric key.

Notable bug fixes:

• keyof any now evaluates to any as expected. Spurious errors related to the bug will no longer fire.
• Fix a server crash ("thread 'flow-tokio-runtime' has overflowed its stack") when rechecking a file with an attached LSP client

Parser:

• Parse component and hook type annotations as the declaration of declare export default

Library Definitions:

• Add ES2026 Map methods

Breaking changes:

• The following flags are removed since they no longer change anything
  • experimental.channel_mode
  • experimental.channel_mode.windows
  • experimental.long_lived_workers
  • experimental.long_lived_workers.windows
  • gc.worker.custom_major_ratio
  • gc.worker.custom_minor_max_size
  • gc.worker.custom_minor_ratio
  • gc.worker.major_heap_increment
  • gc.worker.minor_heap_size
  • gc.worker.space_overhead
  • gc.worker.window_size
  • sharedmemory.hash_table_pow
  • sharedmemory.heap_size

Likely to cause new Flow errors:

• General subtyping errors will print the types that are incompatible. Some errors might have new locations.

No behavioral changes in this release

New Features:

• .flowconfig's [include] and [ignore] sections can now be configured using globs, in addition to the current regex based system. Right now, glob items must be specified with glob: prefix. Globs do not support <PROJECT_ROOT>, since they are assumed to start from the project root.
• Class fields and methods whose computed key resolves to a literal string or number are now supported and type-checked as named members.

No behavioral changes in this release

Notable bug fixes:

• Fixed #9448
• Fixed a crash in pattern matching. (example)
• Refining a union by instanceof no longer leaks any from members that cannot match the guard (e.g. Foo<string> | Bar<number> refined by instanceof Foo now yields Foo<string> instead of Foo<string> | Foo<any>). This may surface real errors that were previously masked.

Parser:

• flow-parser is now backed by the Rust port parser. It will have public API similar to that of hermes-parser.

Likely to cause new Flow errors:

• Object operations on class instance types now produce plain object types instead of preserving the class instance's nominal identity. This affects object type spreads and utilities such as Partial, Required, Readonly, Pick, and Omit. Example

Likely to cause new Flow errors:

• The config for casting_syntax is removed. The as casting syntax is the only supported casting syntax. See Modernizing legacy syntax for migration guidance.
• Flow will now error on T: B in favor of T extends B in type parameters, and error on +/- in favor of readonly/writeonly . See Modernizing legacy syntax for migration guidance.

New Features:

• Support the global NaN as a match pattern

Notable bug fixes:

• Fixed negative bigint literal patterns in match being treated as their positive counterpart
• ConstructorParameters and InstanceType now work on Class<C>, not just typeof C
• match instance patterns whose constructor is a subclass of the scrutinee type are no longer incorrectly reported as unused.

Likely to cause new Flow errors:

• Flow will now error on all deprecated utility types. See Modernizing legacy syntax for migration guidance.

New Features:

• Allow -? (optionality removal) in mapped types.
• Adds support for utility types ConstructorParameters and InstanceType

Misc:

• The .flowconfig options component_syntax, enums, and pattern_matching now default to true. To keep the previous behavior, set the corresponding option to =false in your .flowconfig.

Likely to cause new Flow errors:

• deprecated-type and deprecated-type-bool errors are now always on (the lints were on by default already). Update any references of the type bool to boolean, and delete references to the lints in your .flowconfig and flowlint comments — the setting is a no-op for this release and will be removed in a future release.

• Variance keywords (readonly, writeonly, in, out) on type parameters and object/class properties are now enabled by default. The experimental.allow_variance_keywords is no longer needed to opt in.
• The deprecated variance sigils error message now suggests the contextually correct keyword (readonly/writeonly for property positions, out/in for type parameter positions) instead of listing both alternatives.

Likely to cause new Flow errors:

• Deleted the implicit-inexact-object lint, it no longer does anything since exact_by_default=false was deprecated.
• exact_by_default=false is now deprecated in .flowconfig. Read this blog post on how to migrate, and our docs on objects for more. exact_by_default=true is the default (since 2023) so simply delete the option when migrating.

Notable bug fixes:

• Fixed incorrect enum cast suggestion when using an enum as a type argument

Likely to cause new Flow errors:

• Flow has changed its casting syntax from (x: T) to x as T. The default value of the option casting_syntax is now as, and Flow will error on the old syntax. If you want to preserve the old defualt value, add casting_syntax=both to your .flowconfig. You can use the npm package flow-upgrade to upgade: yarn run flow-codemod typeCastToAsExpression. Read the docs for more.

New Features:

• Exported class setters no longer require an explicit type annotation: void is used if omitted

Parser:

• The flow-parser estree output now aligns more with hermes-parser: ArrayExpression gains a trailingComma field, optional chaining is wrapped in a ChainExpression, AssertNonnull (expr!) emits a NonNullExpression instead of an optional node, type-position this emits ThisTypeAnnotation, and TypeParameter.bound is now a bare type node without the TypeAnnotation wrapper.

• Fix in saved state checks after a rebase

Likely to cause new Flow errors:

• Flow will now error on variance modifiers (+T, -T, in T, out T) on interface method type parameters, matching the existing rejection on class methods and function types

New Features:

• Declaration merging between declare class and interface of the same name in the same scope.
• Flow now separates value namespace and type namespace, so code like const A = 1; interface A {} is now accepted.

New Features:

• Added support for string and number literal properties in declare class and interface

Library Definitions:

• Change the Record<K, T> utility type's key bound from K: string to K: PropertyKey, allowing number and symbol keys in addition to string
• Added some TS built-in types that were missing from Flow's core lib

Misc:

• When experimental.allow_variance_keywords is set to true, we support the keywords in / out / readonly along with +/- for variance

New Features:

• Support constructor type signatures (new (...) => Type) in interfaces and declare class

Misc:

• When allow_variance_keywords is set to true, Flow stops erroring for in / out / readonly

No significant changes.

Parser:

• The 5 enum body ESTree node types (EnumBooleanBody, EnumNumberBody, EnumStringBody, EnumSymbolBody, EnumBigIntBody) have been replaced with a single EnumBody node. Its members property is a flat array of per-type member nodes (EnumBooleanMember, EnumNumberMember, EnumStringMember, EnumBigIntMember, EnumDefaultedMember). The explicitType property is now a string ("boolean", "number", "string", "symbol", "bigint") or null (previously a boolean). hasUnknownMembers remains a boolean. Several enum validation errors are no longer reported by the parser and are instead reported by the type checker.

Library Definitions:

• Add type definition for RegExp.escape()

• Misc: Store exports index in saved state

Notable bug fixes:

• Support numeric literal object keys when importing/exporting object literals
• Allow type keywords like infer to appear as member names in qualified type access (e.g. type S = O.infer;).
• Allow type keywords (readonly, keyof, infer, etc.) as function type parameter names

Parser:

• It is now a parse error to have a ? optional modifier in various places this does not make sense, e.g. const x? = 1;

Misc:

• API change: flow check becomes an alias of flow status. You can use flow full-check to trigger a full foreground check of a Flow root.

• Fix in Meta-internal saved state logic

New Features:

• Support for using well-known symbols as property names in types - the form should be exactly Symbol.iterator.

Notable bug fixes:

• Report prop-missing errors for unbound module properties in explicit type args used with implicit instantiation (e.g. try-Flow)

Library Definitions:

• Updated core.js library definitions to accept readonly tuples in Map constructor

• Fix in saved state on projects including files outside the project root.

Misc:

• Add autofix for colon-style type parameter bounds to extends-style type parameter bounds

Notable bug fixes:

• Fixed analysis of number literal keys in object destructuring rest type computation

New Features:

• Adds a new stylex_shorthand_prop flowconfig option that enables syntactic sugar for StyleX on lowercase (intrinsic) JSX elements. When set to a prop name (e.g. stylex_shorthand_prop=sx), <div sx={[styles.foo, styles.bar]} /> desugars to <div {...stylex.props(styles.foo, styles.bar)} />.

Notable bug fixes:

• Fix https://github.com/facebook/flow/issues/9357

Misc:

• Flow no longer errors on colon-style extends and now supports extends by default, previously gated by the ts_syntax option.
• Add format, and test commands to LLM rules

IDE:

• Fixed https://github.com/facebook/flow/issues/9350

IDE:

• For React autoimport, it will now always be in the form of import * as React from 'react'

Misc:

• Minor changes to the flow-upgrade script

Parser:

• Support export declare, like declare export
• For component in implicit declare context, the ast we produce will include implicitDeclare: true

Likely to cause new Flow errors:

• Imported functions that conform to hook naming conventions will always be treated like hooks. As a result, new errors might be surfaced. e.g. conditional hook call

Parser:

• flow_parser.js now allows you to pass in filename in the option object

New Features:

• declare keyword can be omitted for declare function under declaration context (.flow file, declare module, declare namespace)

Parser:

• Fix parsing of experimental record expressions that are multiline

Declarations:

• The declare keyword can be omitted for declare const/let/var variable: type and declare component under a declaration context (libdef file, .js.flow file, declare module, declare namespace)
• declare component now requires the rest props to be named - matching normal component declarations

Misc:

• The Omit<O, K> utility type will no longer enforce that the provided keys K need to be part of O

New Features:

• Support 4 well-known symbols (Symbol.iterator, Symbol.asyncIterator, Symbol.dispose, Symbol.asyncDispose) in class methods. For now, the computed method name needs to be exactly in the form of Symbol.iterator (or other 3 names) to be recognized.

Notable bug fixes:

• Flow will now error when a function parameter appears in the default expression for the same binding. (e.g. try-Flow)

Misc:

• Introduced a codemod for migrating legacy utility types to modern equivalents (yarn run flow-codemod convertLegacyUtilityTypes)

Parser:

• Flow now parses import attributes. For now, we don't do any semantic validation yet

Parser:

• Support second argument of import expressions

Bug Fixes:

• Fixed bug in computing type hints under JSX expressions (Example of bug fixed)

Config:

• use_mixed_in_catch_variables option was renamed use_unknown_in_catch_variables because mixed is being renamed to unknown

Likely to cause new Flow errors:

• Flow will start to validate variance for declare classes.

Parser:

• Rename ClassDeclaration property 'superTypeParameters' to 'superTypeArguments'

IDE:

• Flow now provides a code action to fix all invalid-type-cast-syntax errors in the file.

Likely to cause new Flow errors:

• flow will now validate variance in interface declarations
• Bad cyclic object types are now banned. e.g. type O_loop = { p: 0, __proto__: O_loop } will cause an error.
• Flow is now stricter with generic mapped type. example

Notable bug fixes:

• We improved the refinement system of match. Some code that previously gets a general type will now get a refined type. example
• allow opaque types to be used as keys to computed object properties
• autocomplete on import { | } from 'a'; will only suggest values, not types.
• flow-dev-tools now properly detects unused suppressions

IDE:

• Flow now provide auto-import code actions for all unbound names, even through they might resolve to a global. It can be helpful when the global is not what you want.

Library Definitions:

• Add bigint functions to DataView

Likely to cause new Flow errors:

• For a poorly bounded generic indexed access types, we will no longer generate an error for it when it's being first subtyped against. Instead, we will make it opaque so that when it's being subtyped against, it will error. example
• Flow will error when you are trying to redefine the following reserved types: $NonMaybeType, NonNullable, $ReadOnly, Readonly, $Keys, $Values, Values, $ReadOnlyMap, ReadonlyMap, $ReadOnlySet, ReadonlySet.

Notable bug fixes:

• We no longer incorrectly report conditional hook call for loops with a break. example
• Fixed a potential crash when evaluating a conditional type. example
• Flow will now allow computed keys of type StringPrefix/StringSuffix to be included in object initializers with other non-overlapping keys (example try-Flow)

Library Definitions:

• Update React API types from recent releases Added

• act: 19.0: https://react.dev/reference/react/act
• captureOwnerStack: 19.1: See https://react.dev/reference/react/captureOwnerStack
• useEffectEvent: 19.2: https://react.dev/reference/react/useEffectEvent
• <Activity />: 19.2: See https://react.dev/reference/react/Activity Updated
• useDeferredValue: 19.0: Accepts a second argument for initial value
• useTransition: 19.0: Accepts an async action function
• startTransition: 19.0: Accepts an async action function

Likely to cause new Flow errors:

• React.ElementProps is removed. You should use React.ElementConfig instead.
• For a function named as a hook but not annotated as a hook, it will no longer be treated like a hook when you try to use it from property access or property destructuring in component syntax components and hook syntax hooks. They will now cause react-rule-hook-non-hook-syntax errors. To fix, annotate them with hook annotation. example
• Calling functions passed to function and component with hook like name but not annotated as hook will now error. example.
• React$RefSetter<...> and and React$Context are removed. It continues to be available in the form of React.RefSetter<...> and React.Context<...>, regardless of whether you have imported React.
• React$ElementRef<...> was removed. It continues to be available in the form of React.ElementRef<...>, regardless of whether you have imported React.

Breaking Changes:

• The config option max_literal_length was removed. Flow will always decide whether to infer a specific or general type of string literal based on the natural inference algorithm.

Notable bug fixes:

• You can now use @@dispose and @@asyncDispose in your library typings. You can access the respective properties with obj[Symbol.dispose] and obj[Symbol.asyncDispose].
• Allow comparisons of bigint singletons

Library Definitions:

• The second type argument of ExactReactElement will be populated by React.ElementConfig<C> instead of React.ElementProps<C>. Most of the code that doesn't depend on exact react element types won't be affected.

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.