flow-parser

Library Definitions:

• Add StorageEvent overloads.

Misc:

• Use textEdit in identifier completion item responses over the LSP, to avoid possible behavior differences with different LSP clients.
• Various improvements to the pretty-printer.
• A small improvement in completeness when handling the "not" operation in the presence of unions of literal types.
• Wrap the usage message to 100 chars.
• Fix a poorly-positioned error related to polymorphic function types.

Parser:

• Improved some parser error messages by treating the end of file as a valid end of object property.
• Updated ESTree output for import expressions to match the spec.

flow-dev-tools:

• Add the update-suppressions subcommand.

New Features:

• Added support for the babel react jsx transform with the react.runtime flowconfig option. When set to automatic the transform auto-imports jsx (the replacement for createElement) under a fresh name (unique of all existing names in the source). When set to classic it will continue transpiling jsx to React.createElement.

Notable bug fixes:

• We are now treating $Exact<T|U> as $Exact<T>|$Exact<U>. Before, when we checked against this type we would treat it like $Exact<T> & $Exact<U>, instead.
• Fix to prevent infinitely expanding recursive type applications of arrays (example).

Misc:

• Improvements in comment attachment in various kinds of AST nodes. These will help improve the accuracy of error suppression and various services that inspect comments (printing, showing documentation, etc.).
• $Exports<'m'> will now lookup module 'm' directly in builtins, instead of the environment.
• Extend union optimizations used on maybe predicate to also be applied in exists predicates.

Parser:

• The range around type cast expression now includes the parentheses. This follows the pattern of other constructs with mandatory punctuation like blocks, objects or arrays.

Likely to cause new Flow errors:

• Moved react-dom modules out of built-in libdefs. They are now available on flow-typed.

  To install the libdefs, run flow-typed install --flow-version=0.122.0 react-dom@16.13.0

  This diff is necessary to allow React to make breaking changes to react-dom without force-pinning Flow users to the latest version of react-dom.

• Fixed an unsoundness in union and intersection spreads

• Improved object spread-related error messages, which may cause suppressed errors to become unsuppressed because they moved to better locations

Improved Editor Integration:

• Removed completion of function param snippets. Before, f<tab> might complete foo(aParam), which is problematic if you don't want to call the function. Now it just completes foo. These snippets are further obviated by signature help, which appears when you type (.
• Improved accuracy of "Did You Mean?" Quick Fixes for object property type errors
• Added support for Go to Definition on export-from declarations (export { HERE } from ...)
• Fixed tracking of open files in LSP clients which could cause stale errors in the IDE

Misc:

• Added HTMLUnknownElement and support for custom elements to document.createElement (thanks @YevhenKap!)
• Fixed --saved-state-no-fallback flag so that Flow exits when saved state is not found
• Fixed an issue where an invalid package.json would prevent generating saved state
• Fixed a potential deadlock when communicating with Watchman
• Fixed types-first signatures for CJS requires introduced with var
• Enabling experimental.types_first now implies experimental.well_formed_exports

Parser:

• Renamed RestProperty to RestElement to match estree
• Changed 'for await' nodes from ForAwaitStatement to ForOf with await: true to match estree

Highlights:

• Made several improvements to errors:
  • Restricted errors to only be suppressible at the error's primary location (see blog post)
  • Fixed the error grouping logic to no longer group unrelated errors that happen to share a location
  • Changed the order of locations printed in error messages so that the primary location is always printed first (which is the one you need to suppress if you want to do that)

Misc:

• Fixed the add-comments script to add comments on JSXText
• Fixed a crash when TMPDIR exceeded 83 characters
• Stopped parsing .flow files without @flow

Lib defs:

• Improved definitions for Node's url.parse function (thanks @chicoxyzzy)
• Added missing console methods (thanks @goodmind)
• Added Element.hasAttributes()
• Added string index to NamedNodeMap
• Fixed Element.querySelector() and Element.querySelectorAll() overloads
• Added type definition for Promise.allSettled()

Likely to cause new Flow errors:

• $Shape types now carry more information about the locations of errors that involve them, which may invalidate old suppressions.
• any-typed values are now refined by primitive typeof checks (example).

New Features:

• "Did You Mean?" IDE feature that suggests corrections to your code as you type. For example, if you write foo.bar on an object foo that doesn’t have a field named bar but does have a field named baz, the quick fix will apply that suggestion.

• declare can now be used on class fields. When Flow implemented the class fields proposal, uninitialized class fields (e.g. class C { foo }) were not allowed, so we used that syntax for type-only declarations (class C { foo: string }). Since then, they now are initialized to undefined and Babel 8 will start to leave them. To maintain existing behavior, change uninitialized fields to use the declare keyword, signalling to Babel that they should be stripped (class C { declare foo: string }).

Notable bug fixes:

• Fixed possible stack overflow from overly long traces.
• Fixed infinite recursion case involving polymorphic types.
• Various improvements to the output of type-at-pos and signatureHelp

Misc:

• Fixed libdefs of Notification to have read-only fields

Notable bug fixes:

• Fixed a bug that would cause Flow to crash on some rechecks.

New Features:

• Implemented LSP textDocument/signatureHelp, which shows parameter hints when you are within an argument list in a call or new expression.

Misc:

• Removed the minimal_merge flag
• Added optional callback for node dgram socket.close function (thanks @davidnaas!)

Parser:

• Made arguments on new expressions optional

New Features:

• Autocomplete for nullable objects will now suggest completions with optional chaining syntax
• Added a new lint, unsafe-addition, which warns if either operand of an addition is null or void

Breaking change:

• The output of flow autocomplete --json no longer includes location information. This might affect some IDE integrations which have not yet updated to Flow's LSP server. The LSP integration is unaffected.

Misc:

• Added the dir field to the Document libdef (Thanks, @lukeapage!)

• Improved timeouts related to Watchman that could cause the server to fail to start if Watchman is slow to respond. Added a file_watcher_timeout .flowconfig option and --file-watcher-timeout argument to flow start and flow server, which defaults to 120 seconds.

Notable bug fixes:

• Improved the behavior of $Diff and other type destructors when applied to unions.

Misc:

• Improved error messages when attempting to use watchman without watchman installed.
• Removed uses of Symbol from libdefs in favor of symbol.
• Fixed definition of fs.promises.mkdir (thanks @gabrielrumiranda!)

Notable bug fixes:

• Fix #8259.

New Features:

• Flow now suggests similar names (if applicable) when issuing missing property errors.

Misc:

• Added a rudimentary libdef for Array.prototype.flat (#8237, thanks @nnmrts!).
• Fixed a bug that in certain specific cases led to an expression getting typed as empty rather than any.

Parser:

• > and } in JSX child text is a parse error.

Likely to cause new Flow errors:

• In Types First mode, exported classes and functions cannot be reassigned.
• Added similar checking of symbol in non-strict equality as other primitive types (example). We allow for non-strict equality checks between two symbol types, and between a symbol and null/void (example).
• We now write properties to unsealed objects recorded during speculative checking, after speculation has succeeded.
• Added more complete handling for indexers in spreads:
  • Use the indexer on the right, when the left is empty, exact and has no indexer. [example]
  • Allow matching indexers to be spread. [example]
• Removed the non-array-spread lint, and replaced it with a new .flowconfig option, babel_loose_array_spread.

Notable bug fixes:

• Improved union optimization for non-enum union equality checks.
• Fixed a bug in which $Exact<T> failed when T resolved to a Shape<T>. [example]

Misc:

• Improved performance of calculating the set of files that need to be checked.
• Cached the typed AST as part of the persistent connection client info, to speed up requests coming from an LSP client (DefinitionRequest, Hover, TypeCoverage).
• Improved dependency graph construction times, by using Hashtbl and mutable back edges.
• Improved tool add-comments by
  • using flowlint-next-line instead of adding full $FlowFixMe suppressions on lints, and
  • making the rule for inserting comments inside JSX elements also apply to JSX fragments.
• Upgraded to sedlex 2.1.

Likely to cause new Flow errors:

• Fixed the order of evaluation of JSX attributes and children to match runtime, so that refinements work properly. This can lead to new errors if refined values are used as children, and a function is called within an attribute, because the refinements are now invalidated in a different order. On the other hand, calling a function in a child no longer incorrectly invalidates values used as attributes. [example]
• Added support for refinements on optional chains! Now, examples like if (x?.y) and if (x?.y === 42) understand that x is truthy in the consequent. Flow also now refines subexpressions of optional chains. For example, given obj?.fun(obj.value), the second obj is known to be truthy since it is only reachable if the earlier obj?.fun is also truthy; therefore, you don't need to optional-chain it again (obj?.value).

New Features:

• Added typechecking support for symbol! Note that just like number is not a Number, and vice versa, symbol is not a Symbol. In the core lib defs, we change usage of Symbol to symbol & Symbol. Eventually, it will just be symbol. The symbol & Symbol is temporary for one version to allow for existing usage of the : Symbol to be codemodded to : symbol.

Bug fixes:

• Optimized typechecking pointwise-equal unions, from O(n^2) to O(n)
• Optimized reverse dependency computation, greatly reducing the time spent computing what needs to be rechecked after an edit
• Fixed an error message that pointed at the definition of a write-only property instead of the callsite that tried to read it [example]
• Fixed an issue where coverage results could be incorrect in Types First mode when a file is rechecked
• Fixed an issue when Flow prints evaluated types like $ElementType, that led to printing empty instead. This improves hover tooltips.
• Fixed several issues causing various processes to die uncleanly while exiting.

Bug fixes:

• Fixed autocomplete when at the end of file
• Stopped filtering lints (e.g. signature-verification-failure) out when running flow check-contents

Parser:

• Added support for export default class implements Foo {} [example]
• Added support for type parameters in anonymous class expressions [example] (thanks @nicolo-ribaudo)

Library definitions:

• Fixed type of navigator.mediaDevices.getUserMedia (thanks @thecotne)
• Added missing InputEvent to removeEventListener
• Updated type of ServiceWorkerContainer.getRegistration

Misc:

• Added module.system.node.main_field option (see #8128 for more details)

Likely to cause new Flow errors:

• JSX expressions now use the new spread semantics added in v0.111.0
• Attempting to write to or update a read-only property using operator assignment is now a type error

New Features:

• New ambiguous-object-type lint warning against use of {} object types (prefer {||} or {...} instead, even when exact-by-default is enabled).
• Improved support for optional chaining

Notable bug fixes:

• Malformed type annotations no longer trigger the unclear-type lint error
• Builtin classes can no longer be extended

Misc:

• Improved performance for utility types and refinements in unions
• Various improvements to autocomplete and get-def IDE services
• Add useDeferredValue and useTransition to React library definitions
• Add bytesWritten function to library definition (thanks @farzonl)

Notable bug fixes:

• 0.111.2 did not include the commit that bumped the Flow version inside of Flow. (@jbrown215 was a clown)

New features:

• Add module.system.node.root_relative_dirname to allow you to configure where root relative paths resolve to

Notable bug fixes:

• Fixed the regex generator that handles the <VERSION> magic string in Flow suppression comments
• Allow spreads of bools, strings, and numbers to support common React Native patterns

Likely to cause new Flow errors:

• Fixes to object spread. See https://medium.com/flow-type/spreads-common-errors-fixes-9701012e9d58

New features:

• Introduced an experimental flag (experimental.minimal_merge) that speeds up rechecks when the experimental types-first mode is in use. This flag will be turned on by default and then removed in future releases.
• Turned on experimental.allow_skip_direct_dependents by default. The flag will be removed next release. This flag speeds up rechecks when the experimental types-first mode is in use.

Notable bug fixes:

• Fixed a bug where the ocaml representation of union types caused crashes in rare scenarios
• Fixed a bug in node module resolution which allowed module.system.node.resolve_dirname=.. For those who relied on this bug to import modules using root-relative paths, you can now use module.system.node.allow_root_relative=true. See #8156 for more details.

Misc:

• Add getElementById to DocumentFragment
• Add missing methods to Blob (thanks @lyleunderwood!)
• Allow clients of flow status, flow check, etc. to choose character offset style
• Support length refinement on tuples (thanks @ilya-bobyr)

Parser:

• Allow => in objects in return types of arrow functions

Notable bug fixes:

• No longer show live Flow errors for files without @flow, unless all=true in the .flowconfig
• No longer show live Flow errors for ignored files
• Re-generate live errors after a recheck

New Features:

• Flow will now send type errors as you type to LSP clients. To disable this behavior, add experimental.disable_live_non_parse_errors=true to the [options] section of your .flowconfig.

Notable bug fixes:

• Fixed file descriptor leak leading to LSP connection refusal in some cases
• Improved the behavior of the [declarations] configuration in the presence of dependency cycles (thanks @STRML!)

Misc:

• Added --evaluate-type-destructors to type-at-pos command.
• Added --evaluate-type-destructors and --expand-type-aliases to dump-types command (thanks @goodmind!)
• Changed process.env values from ?string to string|void (thanks @FireyFly!)
• Improved detection of rebases when using watchman file watcher
• Improved positions for error messages involving the deprecated * type

Likely to cause new Flow errors:

• Flow was previously not typechecking delete, but now does
• Object.defineProperty and similar methods now adhere more closely to the spec
• Allow defaults for properties that may not exist in React components

Notable bug fixes:

• $NonMaybeType<mixed> no longer includes null and undefined (Thanks @goodmind)

Misc:

• Deprecated $Supertype and $Subtype utilities are now removed entirely
• flow-upgrade should now use https over git (Thanks @lukeapage)
• Autocomplete now fires on spaces in JSX
• Better error messages when a value is used as a type

Performance:

• Reduced memory usage by using a more compact representation for code locations
• Types-first should no longer check direct dependents of files where the signature has not changed

Library Definitions

• Add a number of CSSOM interfaces + fix HTMLStyleElement.sheet type (Thanks @kof)
• Add AsyncIterator to node.js (Thanks @goodmind)
• Add undefined to prelude (Thanks @goodmind)
• Add String.prototype.matchAll (Thanks @goodmind)

Notable bug fixes:

• Batch coverage info now persists through rechecks.
• When a file with @preventMunge is in a cycle with a file that does not, the un-munged file no longer has exported munged properties checked for annotations.
• Fixed a bug where autocomplete would not be triggered in a file with Windows-style line endings.
• Improved error messages for uses of $ObjMap, $ObjMapi, $TupleMap, and $Call with incorrect arity.

Misc:

• The deprecated flow ide command and associated machinery have been removed.

Library definitions:

• Added the missing parts of the Pointer Lock spec to libdefs.
• Thanks to @lyleunderwood for adding scrolling support to libdefs.
• Thanks to @goodmind for adding a definition for undefined to the prelude.

New Features:

• Implement type refinements for property accesses through brackets (#7597, thanks @goodmind).

Notable bug fixes:

• Fix several issues with autocomplete that prevented optional properties, type aliases to utility types, types with default type arguments, and several other edge cases from being autocompleted.
• Fix Not_expects_bounds crash with %checks (#7863).

Misc:

• Make the LSP textDocument/documentHighlight request serviceable while Flow is in the middle of a recheck.
• Fix minor off-by-one error which caused some parts of traces to be pruned when using the --traces flag.
• A minor improvement in error messages when undefined is involved.
• Reduce memory usage slightly when abstract locations are enabled.
• Prevent log spew when flow lsp is started while the server is initializing.
• Improve completeness of sighashing. We recently observed an incremental bug caused by incomplete sighashing. This speculatively addresses similar potentially problematic cases.

Library Definitions:

• Add webkitGetAsEntry to DataTransferItem.
• Switch several properties to optional in Notification and NotificationOptions (#8032, thanks @pauldijou).
• Map/Set fix symbols (#7560, thanks @goodmind).
• Fix Array#reduce and Array#reduceRight (#7902, thanks @goodmind).

We found and fixed a bug introduced in 0.105.0. Some internal code was using a hashing function and assumed collisions were far less likely than they proved to be. This could lead to random nonsensical errors which would then disappear, usually involving missing object properties. This likely only affected extremely large projects.

Fixed the stack overflow reported by #8037. Thanks @lukeapage for isolating the repro!

Forgot to cherry-pick [rollouts] (an experimental new .flowconfig section) into v0.106.0

Likely to cause new Flow errors:

• We're starting to make changes to how Flow models object spreads. For more see this announcement
• Updated parsing of the experimental nullish coalescing ?? operator. It now has a lower precedence than || and &&, and parentheses are required to nest it with them.
• Flow wasn't typechecking the properties of certain obscure JSX usage (namespaced identifiers, member expression with @jsx / @csx), so would miss type errors in their expressions (e.g. <a:b prop={"hello" * 10} /> now errors)

Notable bug fixes:

• Fixed a bug where merge or check jobs would crash when a parse error was added to a file in a cycle. The crash was silent but unintended.
• Types-first no longer ignores the munge_underscores flowconfig option

Misc:

• Various libdef updates. Thanks for all the PRs!

Parser:

• Improved error messages for missing semicolon
• Comments are now correctly attached to break statements and array patterns
• libflowparser now supports esproposal_nullish_coalescing as an option

v0.105.0 started running the Flow server in a cgroup on Linux distros that support cgroup v2. However, some versions of systemd, which manages cgroups, contain a bug that caused the Flow server to fail to start. This release avoids using cgroup on these systems. (#8012)

Likely to cause new Flow errors:

• Types for FileReader properties and methods are now more precise (e.g., some parameters typed any are now typed ProgressEvent; some properties now have null added to their types). Thanks, @nwoltman!

• The value type parameter V of $ReadOnlyMap and $ReadOnlyWeakMap is now covariant. Thanks, @goodmind!

• Types for the vm module in node.js are now more precise. Thanks, @goodmind!

• The deprecated $Enum<...> utility type has now been deleted. Use $Keys<...> instead.

• Indexing tuples with floats is no longer allowed.

New Features:

• Added support for React.Profiler (React v16.9+). Thanks, @bvaughn!

• Added a --types flag to flow graph dep-graph to output only "type" dependencies: the subset of imports that the types of a module's exports depends on. (Without the flag, we output "code" dependencies: the set of all imports of a module.)

• Preliminary support for automatically inserting annotations on a module's exports through LSP. Thanks to @akuhlens (summer intern with the Flow team)!

• Preliminary support for definite assignment checking of class instance properties. Thanks to @pzp1997 (summer intern with the Flow team)!

• Added an option to .flowconfig for exact-by-default objects.

Perf fixes:

• Fixed a non-termination issue with a recursive use of mapped types.
• Fixed an exponential-blowup issue with a combined use of spreads and unions.
• Fixed an exponential-blowup issue with recursive use of array spreads.

Misc:

• Fixed LSP init to say codeLens is not supported.
• Fixed lots of cases of bad error positioning, unblocking improvements to error suppressions and error streaming. Thanks to @mvcccccc (summer intern with the Flow team)!

Parser:

• Improved a bunch of "unexpected" parse errors, providing what was expected in the error message.
• Fixed a bug in parsing of params in function types.

Likely to cause new Flow errors:

• Fixed the definition of Function.prototype.apply to only accept array-like objects, not any iterable.
• Improved error positioning, which may cause previously-suppressed errors to become unsuppressed.

New Features:

• non-array-spread lint rule: Fires when a non-array iterable is spread. This is useful for modeling the loose: true mode of @babel/plugin-transform-spread, where such code causes a runtime error.

Notable Bug Fixes:

• Fixed a performance regression when computing dependencies in large projects
• Fixed built-in library definitions that needed to be explicitly inexact to pass the implicit-inexact-object linter.
• Improved libdefs for many browser APIs (e.g. Media Streams, MIDI, Permissions, Workers) (#7737, #7805, #7806, thanks @goodmind!)

Misc:

• Various improvements to the types-first signature generator
• Improved the name of the server master process in ps
• Improved the output of flow check --profile

flow-remove-types:

• Remove opaque types and declare export

Parser:

• Fixed a bug allowing await to be a parameter in async functions
• Several improvements to the experimental comment attachment algorithm

New Features:

• Added --types flag to flow cycle, when given only type dependencies are used to compute cycles

Notable bug fixes:

• Fixed a bug when destructuring unions gave spurious errors [example]
• Updated for-in/for-of head expressions to be evaluated in the correct scope [example]

Performance:

• Improved the calculation for what are dependents of a file, reducing work during rechecks

Library definitions:

• Added support for Array#flatMap (thanks @goodmind)
• Replaced usages of Object and Function (which are aliases for any) in library definitions with any
• Removed some usages of any from library definitions
• Updated Function#apply to accept strictly two arguments (thanks @goodmind)
• Added SpeechSynthesis definitions (thanks @goodmind)

Misc:

• Updated/added HTML spec URLs in comments (thanks @kevinSuttle)
• Fixed parsing of anonymous class implements clause (thanks @goodmind)
• Added support for printing mixins and implements (thanks @goodmind)

Likely to cause new Flow errors:

• Function components with no arguments get a sealed empty object type as props.
• Moved MixedElement export into the module declaration, so it will now need to be qualified as React.MixedElement.

Notable bug fixes:

• Fixed error positioning around utility types (e.g. $ObjMap).
• Omit reporting error stack traces to end users over LSP.
• Fixed bug where Flow would crash when variable has same name as a type (fixes #7825)

Misc:

• Refactored coverage computation to use the typed AST. This enables coverage results over more locations that earlier.
• Improved server and monitor error logging.
• In typing object types as react components, account for the defaultProps property and make them compatible with React.AbstractComponent.
• Optimized the way module exports are populated to prevent recursion limiter exceptions.
• Improved error messages for invalid BigInts. (thanks, @goodmind!)
• Hovering over an imported type alias returns its definition. (thanks, @vicapow!)
• Fixed semver comparison to allow for suffixes such as rc.

Libdefs:

• Remove Object type (equivalent to any) from WeakSet and $ReadOnlyWeakSet. (thanks, @goodmind!)
• Add methods to Node HTTP ServerResponse type definition. (thanks, @chrislloyd!)
• Add definitions for the Web Animations API. (thanks, @goodmind!)

Notable bug fixes:

• Fixed a bug with suppressions in the experimental types-first mode.

Likely to cause new Flow errors:

• $Keys now produces a more precise type, which may find errors where incompatible strings were passed to something expecting the $Keys of some object.

New Features:

• We released a new implicit-inexact-object lint to detect when an inexact object is used without explicitly adding ... to the end of the props list. See here for context.
• Function type parameters may now use default arguments. This is not yet supported by babel.

Notable bug fixes:

• Fixed a bug with ranges returned by autocomplete
• Fixed a bug where errors with bad locations reported over the LSP could cause the editor to clear all errors.

Misc:

• React.memo and React.lazy now both allow you to specify an instance type via React.AbstractComponent.
• Various performance improvements to union types.
• Various libdef fixes and improvements.
• Various improvements to error positioning.
• The recursion limit is now configurable in the .flowconfig via recursion_limit. Most projects will not need to override this value.

Parser:

• Forbid private fields named #constructor
• Fix duplicate private class field validation for getters/setters
• Fix parsing of private getters and setters
• Function type parameters may now use default arguments.

Likely to cause new Flow errors:

• The React$ElementType annotation, which was previously unsafe, is now strict. Before you could create an element given a component with this type using arbitrary props. To annotate any component that accepts some given props, use React$ComponentType instead. Try Flow example
• The React$ComponentType annotation is now strict when used with refs. Before, it was possible to pass a ref having any type when creating an element from a component using this type. If you need to describe components that accept refs, use the React$AbstractComponent type instead. Try Flow example
• The $Enum built-in type annotation is now deprecated. Please use the semantically equivalent $Keys type instead.
• Destructuring patterns could previously include missing properties if the resulting binding was unused. This is now an error even when unused. Try Flow example

New Features:

• You can now use the built-in type React$MixedElement as the sound superclass of all React elements. This is a type alias for React$Element<React$ElementType>.

Misc:

• Add decode method to HTMLImageElement (thanks, @vicapow!)

Parser:

• Handle NonOctalDecimalIntegerLiteral
• Remove U+180e (Mongolian vowel separator) from list of valid whitespace code points
• Remove support for legacy octal literals with numeric separators
• Remove support for legacy octal bigints
• Fix various issues related to automatic semicolon insertion (ASI) for class properties

Notable bug fixes:

• Fix bug where well-formed-exports errors were reported for unchecked files

Likely to cause new Flow errors:

• The statics of function types used to be any but are now typed as an empty object.
• Recursive calls of named function expressions were previously unchecked, but are now checked.
• $call property syntax, deprecated in Flow v0.75, has finally been removed.

Notable bug fixes:

• Fix an issue where Flow would not catch certain errors involving React function components with unannotated props.
• Fix React synthetic mouse events for drag, wheel, pointer events to give the specific native event type. (Thanks, @Kiwka!)

Misc:

• Improved performance of starting a server from a saved state.

Parser:

• Fix parsing of function types inside tuples inside arrow function return types.

Notable bug fixes:

• Do not report bad module uses in unchecked files

Likely to cause new Flow errors:

• Infer void before typechecking starts for functions without a return statement, lessening the impact of a union typechecking bug (#7322).
• Fix a bug which prevented Flow from asking for required type annotations.
• Turn the deprecated-utility lint on by default.
• Two related changes to type refinements to fix unsoundness:
  • mixed refined to an array produces a read-only array.
  • mixed refined to an object produces a read-only object.

New Features:

• Add the ability to exclude paths included by a previous pattern in a .flowconfig (#7317).

Notable bug fixes:

• Fix a bug that led IDEs to report all code as uncovered (#7654).
• Fix the untyped-import lint rule so that export [type] * from triggers it.
• Flow now recognizes refinements against negative number literals.

Misc:

• Exclude deprecated-utility and dynamic-export lints when applying all=setting rules (#7473).
• Improve client/server version mismatch behavior so that the newest of the two is preserved, rather than the client version.
• Preserve exactness of the input type when using $ObjMap or $ObjMapi (#7642).
• Minor changes to metadata in the results of flow type-at-pos --json.
• Batch DidOpen notifications from the IDE in order to make checking in IDE lazy mode more efficient.
• When flow lsp automatically starts a server, it prefers the lazy mode set in a .flowconfig to the lazy mode passed on the CLI.
• Allow lints to be explicitly set to their defaults (normally redundant lint settings are disallowed).
• Fix spurious missing annotation errors when the this type is used incorrectly.
• Fix a bug that made React.Element behave differently than React$Element.
• Fix an edge case where object property assignments were typechecked incorrectly (#7618).
• Fix an unsoundness with addition or logical operators when combined with generics (#6671, #7070).
• Fix an issue which allowed read-only arrays to be written to if the index was of type any.
• Fix a bug which stymied typechecking after try/catch blocks (#7530).

Libdefs:

• Add document.elementsFromPoint() (#7540).
• Add ConstantSourceNode (#7543).
• Remove React.Suspense maxDuration attribute (#7613).

Likely to cause new Flow errors:

• Refining a variable of type mixed with instanceof A produces type A instead empty which was produced before.
• Types imported in a declare module are no longer automatically exported from that module as well.

New Features:

• #7518 Adds support for LSP function parameter completion (thanks @vicapow)

Notable bug fixes:

• Return a better error message when flow coverage is passed an invalid input path.
• Fixed a bug in which Flow crashed on very long directory paths.
• Fixed type-at-pos results when reporting the type of a callable object.

Misc:

• Improvements in AST utilities: The differ got improved support in several kinds of type annotations (literals, generic identifiers, typeof, tuples and interface types). The mapper got support for qualified identifiers.
• Introduces a resizable array data structure that is used in union-find.
• Improved error messages around callable and indexer.
• Type-at-pos now shows results without evaluating type destructors like object spread, $Diff, etc. This should lead to more compact results.
• Various refactorings in the internal type language and environment.

Library definition improvements:

• Updates in Node definitions. The stream definitions were updated to the latest version, and the readline.createInterface definitions were also updated.

Parser:

• #7471 Adds support for parsing of BigInt (Arbitrary precision integers) (thanks @goodmind)

• Object literals with spreads can be described by object types with spreads in the signature verifier/builder.

• The inferred statics object type of React.createClass({}) will contain defaultProps: void, instead of defaultProps: {||} (unsealed empty object).
• Bug fix in internal cache mechanism

• Added an overload for JSON.stringify allowing mixed input, which returns string | void. Without this, you can't call JSON.stringify on a mixed value at all, because while Flow does allow refining mixed to "not void" (e.g. x === undefined ? undefined : JSON.stringify(x)), it does not support refining mixed to "not a function" (e.g. imagine you could do x === undefined || typeof x == 'function' ? undefined : JSON.stringify(x)). This rolls back some of the more restrictive behavior introduced in v0.95.0, but is still more restrictive and more accurate than in <= v0.94.0.

Likely to cause new Flow errors:

• Disallow undefined and functions in JSON.stringify: JSON.stringify(undefined) returns undefined instead of string. Rather than make it always return string | void, or use overloads to return void on those inputs, we instead disallow those inputs since they are rarely the intended behavior. (#7447)

New features:

• flow batch-coverage: A new command to compute aggregate coverage for directories and file lists. Instead of producing the coverage at each location, it sums them and reports the per-file percentage and the aggregate percentage.

Bug fixes:

• Fixed incorrect reporting of signature verification lint errors in unchecked files

Other improvements:

• #7459 Add type for Symbol.prototype.description (thanks @dnalborczyk)
• #7452 Add types for String.prototype.trimStart/trimEnd (thanks @dnalborczyk)
• #7500 The "kind" of an autocomplete result is now reported over the Language Server Protocol, improving the autocomplete UI (thanks @vicapow)

Bug fixes:

• Fixed dynamic-exports lint's spurious errors on exported classes and functions
• Handle package.json files that are valid JSON but invalid packages

Performance:

• Reduce memory usage by filtering suppressed lint errors before formatting the errors for printing
• Quicker responses to cancellation requests

Many libdef fixes and other improvements from the open source community:

• #3209 Fix autocomplete for generic type aliases (thanks @vkurchatkin!)
• #6750 Remove shadowed generics in Proxy$traps (thanks @talbenari1!)
• #6000 Document async function return type (thanks @callumlocke!)
• #7448 Tweaks to built-in http module (thanks @STRML!)
• #4570 Update types for Web Audio API (thanks @fand!)
• #5836 Fix examples in libdefs/creation page (thanks @tomasz-sodzawiczny!)

Additional lib def improvements:

• Make current write-only in React.Ref - allowing union types for ref
• Add setMediaKeys API to definition of HTMLMediaElement
• Make type parameter to http$Agent covariant

Likely to cause new Flow errors:

• Removed a constraint involving any types and React proptypes for efficiency. This may result in some errors no longer being reported.

New Features:

• A new lint (dynamic-export) which when enabled will warn when most dynamic types are exported from a file.
• Flow now distinguishes between any and empty when computing line coverage. empty types are colored blue and any types red when using the --color option. Note that this may cause new expressions to be considered uncovered.

Notable bug fixes:

• Fixed a non-termination condition during this-substitution.
• Fixed an issue where inexact-spread lint errors could appear in the wrong position.

Many, many libdef fixes and improvements! Many thanks to the open source community for these, and to @nmote and @jbrown215 for reviewing and merging so many of these!

• #4388 add missing InputEvent(Listener|Handler|Types|) (thanks @keithamus!)
• #4664 Fix IntersectionObserver constructor definition (thanks @apostolos!)
• #4858 Make ServiceWorkerMessageEvent extend ExtendableEvent (thanks @keyiiiii!)
• #4772 add indexer property to string lib def (thanks @zacharygolba!)
• #5529 Add module.builtinModules to core libdef (thanks @simenB!)
• #5574 fix return parameter for writable.setDefaultEncoding() (thanks @dnalborczyk!)
• #5578 add util.callbackify to node type def (thanks @dnalborczyk!)
• #5628 Add lib declaration for BroadcastChannel (thanks @schmatz!)
• #5866 Add definition for timingSafeEqual() (thanks @rolftimmermans!)
• #5988 add destroy method to streams (thanks @hiikezoe!)
• #6091 Fix static declarations for XMLHttpRequest (thanks @robin-pham!)
• #6339 Fix parent of AnimationEvent (thanks @ngyikp!)
• #6367 Add types for Object.getOwnPropertyDescriptors (thanks @disnet!)
• #6471 Actualize node's EventEmitter API definition (thanks @antongolub!)
• #6535 add Element.prototype.toggleAttribute (thanks @keithamus!)
• #6614 Add TransitionEvent to dom libdef (thanks @koddsson!)
• #6785 Allow specifying encoding as string in options field of appendFile, appendFileSync (thanks @cappslock!)
• #6963 Add Audio declaration (thanks @vldvel!)
• #7011 Use more specific type for navigator.serviceWorker (thanks @dhui!)
• #7097 Add type definitions for message events (thanks @wachino!)
• #7122 Support for Uint8Array (thanks @cakoose!)
• #7144 Updated URL modules definitions for Node.js 10 (thanks @MrFranke!)
• #7146 Fix type definition of Node.js dns.lookup() (thanks @shuhei!)
• #7215 fix https interfaces (thanks @cakoose!)
• #7225 make createContextualFragment return a DocumentFragment (thanks @wincent!)
• #7342 add Document.queryCommandSupported (thanks @Eazymov!)
• #7358 Add oncontextmenu to HTMLElement (thanks @jasonLaster!)
• #7363 Add MediaDeviceInfo declaration (thanks @ea167!)
• #7367 Add userSelect to CSS declaration (thanks @shubhodeep9!)
• #7368 Fix fs.promises.readFile being incorrectly overloaded (thanks @Macil!)
• #7381 add EventSource to dom libdef. Likely to cause new errors (thanks @SlIdE42!)
• #7386 fix XDomainRequest in bom libdef. Likely to cause new errors (thanks @Mouvedia!)
• #7387 Added optional displayName property to React$Context (thanks @bvaughn!)
• #7405 Basic support for typeof x === 'symbol' (thanks @mroch!)
• #7420, #7430 and #7440 Various React improvements (thanks @threepointone!)
• #7423 make useRef type non-nullable (thanks @trysound!)
• #7445 add Stream type to Node thanks (thanks @goodmind!)

Misc:

• Updated our website and GitHub issue template to make it easier for open source users to contribute to Flow!
• Various improvements to the AST, including the differ, typed AST and the AST mapper

Notable bug fixes:

• Fixed a bug introduced in v0.92.0 which could cause the server to crash when using an IDE.
• Fixed module.system.haste.name_reducers option for Windows (#7419 - thanks jamesisaac)

Likely to cause new Flow errors:

Some fixes to tagged template literals will surface a new set of pre-existing errors!

New Features:

This release culminates months of hard work on quality of life improvements for IDE support. Expect your requests to be faster, and your requests to take a bit less time.

• Several Flow commands can run in parallel now, i.e. you can still get type definitions while rechecking. Big props to @glevi for this massive QoL improvement!
• Alongside this change, @glevi also released a new lazy-check mode that leverages watchman to reduce the number of checked files. Learn about it in the docs.

Notable bug fixes:

Thank you to the opensource community for these fixes!

• #7354 Fix MouseEvent type definitions
• #7262 Update types for WeakMap
• #7100 Add missing crypto.randomFillSync and crypto.randomFill methods
• #7356 Add definitions for new debug hook useDebugValue
• #7352 Rename React hook useImperativeMethods -> useImperativeHandle
• #5665 Fix arity of clearInterval, clearTimeout
• React.memo now accepts any kind of Component

Likely to cause new Flow errors:

• Better positioning for React error messages. This may move already existing (and suppressed) errors to new positions.

New Features:

• The --lazy-mode flag was added to all commands that may autostart a server

Notable bug fixes:

• Fix a crash when using private class fields (fixes https://github.com/facebook/flow/issues/7355)

Misc:

• Various additions and typo fixes on flow.org docs and README (thanks @fschindler, @dominicfraser, @keithamus, and @fterh)
• Misc OCaml tweaks (thanks @rvantonder)
• Huge reduction (~15%) in total memory usage (thanks @nmote)
• Huge reduction in error collation time (thanks @panagosg7)

Likely to cause new Flow errors:

• Removed unsafe rule allowing Date instances to be used as a number
• Changed $Shape<> types to reject null and void as subtypes
• Removed unsafe refinement from mixed to a function type using typeof

Pull Requests:

• #7290 Add support for Path2D constructor arguments (thanks @zpao!)
• #7221 use package that works with 7.x in babel doc (thanks @rob2d!)
• #7231 Improve type of 'mkdir' and 'mkdirSync' (thanks @mrtnzlml!)
• #7278 fix: update toc links in hoc pages (thanks @evenchange4!)

Notable bug fixes:

• Fixed issue where errors involving $Shape<> types were positioned poorly

Parser:

• Fixed decoding of html entities at beginning of JSX children
• Fixed offset calculation in estree output to account for multibyte characters

Likely to cause new Flow errors:

• Big revamp to React typing with the goal of adding support for React.forwardRef and better typing higher-order components. Docs are available here.

New Features:

• New deprecated-utility lint complains about deprecated Flow types. To start off, $Supertype and $Subtype are now deprecated. They were unsound and can usually be replaced using shapes or bounded generics.
• React.AbstractComponent makes it way easier to type React higher-order components.
• React.Config is also intended to help type React higher-order components.

Notable bug fixes:

• flow coverage --color handles multi-byte characters better
• flow coverage now supports --strip-root

Misc:

• We've deleted flow gen-flow-files due to bitrot. We do plan on building a better version in the future.
• Various libdef updates. Thanks for all the PRs!
• {} now consistently represents an unsealed object. You can read more in the documentation. example

Likely to cause new Flow errors:

• Made Function and Object types be aliases for any. They were always unsafe types, just like any, but they had peculiar behavior. This change revealed places where they were handled improperly within Flow, and ended up surfacing type errors that were previously missed.

New Features:

• Added the experimental Watchman lazy mode (flow start --lazy-mode watchman). This improves the lazy mode experience for repositories which use Mercurial and Watchman. We will document it more when/if it proves itself.
• Added flow config check which validates the .flowconfig.

Misc:

• Made miscellaneous improvements to the AST differ, which improves the output of global rename.
• Made .flowconfig parsing less strict (in particular, if the --ignore-version flag is passed, do not fatal on unrecognized config options).
• Performed a code cleanup in type normalization that caused some types in type-at-pos to be displayed differently.
• Removed redundant information in stored ASTs resulting in a modest reduction in memory usage.
• Flow assigns long string literals type string, rather than the singleton type of that literal. Now, this fact is surfaced in error messages.
• Fixed stack overflows:
  • When checking a large number of files.
  • When a large number of errors are present.

Libdefs:

• Added React.Suspense.
• Removed React.useMutationEffect hook.

Likely to cause new Flow errors:

• Fixed an error in the React.createRef definition. Refs are for any type of value, not just React elements.

New Features:

• Added support for subcommands to the CLI and a flow config find command that finds the .flowconfig governing path.

Notable bug fixes:

• Fixed file_sig for deep destructured requires.
• Fixed a stack overflow in the flow ls --json command.
• Fixed a crash when the module reference prefix is used on an untyped module.

Misc:

• Removed the flow port command.
• Various improvements to the AST differ.
• Made progress in shifting from concrete to abstracting locations, by
  • functorizing a number of modules over their use of locations, and
  • updating the core typechecking logic to operate solely on structures containing abstract locations.
• Removed the redundant Expression.Member.computed field from the AST.
• Allow configuring path to node in runtests.sh.
• Refactored JSX.frag_closingElement of the AST to not be an option.
• Fixed error localization in import type and import typeof with default.
• Added the location of function signatures to the AST.
• Removed prototype members when autocompleting JSX props.
• Cleaned-up handling of the mixed case from LookupT.
• Optimized the case of enumerations when used as keys.

Library definition changes:

• Made geolocation PositionOptions optional.
• Added flow definitions for React hooks.
• Added a calculateChangedBits parameter to the React.createContext definition.
• Added React.ConcurrentMode and React.StrictMode definitions.
• Added React.lazy and React.memo definitions.

Parser:

• Refactored flowconfig option parsing into a list of parsers.

Likely to cause new Flow errors:

• New errors may arise from generic definitions which lack the necessary annotations. Flow does not infer generic types, and the types it used to infer in their place occasionally masked errors. The types it now infers are still not generic, but will mask fewer errors.
• Fixed bug in union type checking which caused Flow to miss some errors when used in conjunction with generics and certain utility types.
• Improvements to constraints involving any may result in new errors.

New Features:

• type-at-pos can reconstruct spread objects when evaluation of the spread is not possible.
• type-at-pos now supports implicit instantiation with _.
• Added core types $ReadOnlyMap, $ReadOnlyWeakMap, $ReadOnlySet, and $ReadOnlyWeakSet. These types and their mutable counterparts mostly follow the pattern of $ReadOnlyArray and Array: the read-only parent class provides non-mutating methods, and the usual mutable class extends it. Unlike $ReadOnlyArray, the new types are invariant in their type parameters.
• Added the React.StrictMode type.
• Added the flowtest package. flowtest is a CLI tool for running tests on Flow types.
• Added the sharedmemory.heap_size option.

Notable bug fixes:

• Restructured file signatures to support overloading in exports.
• Allow named exports in exact module.exports objects.
• Fixed forward references for declare function.
• Various bug fixes to the type normalizer and to type-at-pos.
• flow suggest no longer outputs <<anonymous class>>.
• Imported symbols should no longer appear Remote in type-at-pos.

Misc:

• Improved formatting in JavaScript output.
• Improved the way the element type of an array is determined.
• Various improvements to the AST differ.
• WeakMap keys must now be objects.
• Improved profiling for error collation and formatting.
• When a union lower bound flows into a union upper bound, if both unions are enums, we use the underlying set representation to handle this in O(n log n) time instead of quadratic time.
• The type normalizer properly represents utility types instead of treating them as generics.
• The type normalizer provides more accurate information about generics.
• Flowing a string lower bound into a union upper bound occurs in O(log n) instead of O(n) when the union is an enum.
• CJS modules' namespace objects are now treated as covariant.
• The type normalizer reconstructs literal types more precisely when preserve_inferred_literal_types is set. This does not change the behavior of type-at-pos.
• Deleted the experimental.cancelable_rechecks option.

Parser:

• Removed the deprecated expression field from Function nodes in the AST.
• Enabled some tail call optimizations in flow_parser.js which should cause it to stack overflow in fewer cases.
• The layout generator no longer prints empty statements as {} instead of ; in pretty mode.
• Allow anonymous function parameter types inside generics inside arrow function return types. For example, we disallow var x = (): (string) => number => 123 because the first => is ambiguous. However, var x = (): T<(string) => number> => 123 is not ambiguous and we no longer disallow it.

Likely to cause new Flow errors:

• Fixed an issue that caused missing annotations errors to be suppressed.

  Please read the full post for more information on what changed and tips on dealing with the new errors.

Likely to cause new Flow errors:

• Earlier, type constraints between any and other types would be dropped. Instead, they are now propagated. In some cases, this unblocks further constraint solving and helps find more errors.
• When a variable is equality-checked with a literal, the variable's type is refined. Earlier, if the variable's type was incompatible with the literal's type, it would silently be refined to empty, whereas now this is an error.

New Features:

• Added support for wildcard (_) type arguments to function / constructor calls. This is especially useful when some type arguments are sufficient to pin down the type of the result; the others can simply be _.

Notable bug fixes:

• Fixed a case that would crash get-def and find-refs
• Fixed a bug with unreachability analysis for ternary expressions
• Fixed a bug with refinements merging at the end of switch statements

Misc:

• Fixed various AST printing / layout bugs
• Made various improvements to the AST differ
• Refactored parts of get-def and find-refs to reuse code
• Made progress on abstracting locations in the core type inference engine
• Made progress on module signature verification and generation
• Merged PRs that improve type declarations: e.g., getBoundingClient returns a DOMRect instead of a ClientRect.

Parser:

• Support for ... to indicate inexactness in object types

Likely to cause new Flow errors:

• Fixed a bug where type precision was lost at module boundaries

Notable bug fixes:

• Prevented an exponential blowup when union types flow into "maybe" (?T) or "optional" (essentially T | void) types
• Allowed {p:T} to be a subtype of $Shape<{+p:T}>

Misc:

• Fixed exception when using --traces
• Changed --verbose to not log while loading flowlibs by default; pass --verbose-flowlibs to override
• Added ability for LSP clients to cancel previously-sent commands
• Improved location of diagnostics via LSP while typing
• Fixed LSP textDocument/definition response when there are no results (prevents bogus jump-to-definition inside comments, for example)
• Limited number of errors returned via LSP to 200, to improve Nuclide and Visual Studio performance
• Fixed an exception when attempting to focus a file that Flow ignored

Library definition changes:

• Added ResizeObserver declarations
• Added missing net$Server.listening property
• Added process.cpuUsage()

Parser:

• Fixed export default async function to be a FunctionDeclaration not a FunctionExpression
• Made instance properties named 'constructor' a syntax error