v0.338.0

Breaking Changes:

• Flowconfig option experimental.assertion_functions is removed. Assertion functions have been on by default and the option only accepted true since 0.337.
• Flowconfig option experimental.disable_invariant_special_casing is removed. It has been on by default and only accepted true since 0.337.
• Flowconfig option experimental.function_and_object_type_is_no_longer_any is removed. It has been on by default and only accepted true for a while.
• Flowconfig option experimental.builtin_ts_libdef_dynamic_discovery is removed. It has been on by default and only accepted true.
• Flowconfig option experimental.declare_global_support is removed. It has been on by default and only accepted true.

Likely to cause new Flow errors:

• Readonly, Partial, Required, $Exact, Omit and mapped types over an interface now include its methods and the members it inherits.
• Type guards now refine Array<any>, {data: any}, Map<string, any> and similar types to the guarded type, e.g. Array<string>, which can surface errors in code that relied on any.
• Static members can no longer reference the type parameters of their class, like in TypeScript, and reading a static member of a generic class no longer infers them.
• keyof of an object with a template literal indexer no longer widens the indexer key to string in conditional types like Exclude.

New Features:

• Mapped types over arrays and tuples support readonly and writeonly modifiers.
• Property truthiness checks such as if (result.done) now refine unions of classes and interfaces by fields with literal types, as === checks already did.
• Type guards narrow through optional calls such as isString?.(x). When the callee or receiver is nullable, only the true branch narrows.
• Support optional assertion calls when their declared types guarantee the call runs.
• Added the intrinsic ExactObjectType, the supertype of every exact object type and a subtype of {...}.

Notable bug fixes:

• Fix crashes and false recursive-definition errors from cyclic type star exports in TypeScript declarations.
• Support namespace export aliases and ambient exported members in TypeScript definitions.
• Allow classes to extend a constructor whose overloads are not all generic, such as Uint8Array<ArrayBuffer>.
• Casting a value whose type intersects an enum with other types (e.g. E & E) to the enum's representation type no longer errors.
• Arrays can be assigned to variables and parameters whose type was inferred from new Array<T>(...).
• Report delegated yield type errors at the yield expression.
• Align Try Flow's default lints and declare global support with regular Flow.
• Improve inference for generic values passed to polymorphic operations.
• Fix Try Flow initialization after the async builtin migration while preserving older releases' library definitions.
• Report conditional type instantiation errors at the caller.
• Apply type-parameter defaults when omitted optional or nullable arguments are the only evidence for them.
• An object with an indexer is no longer checked against optional properties it doesn't declare of a TypeScript-declared (.d.ts) object type, matching TypeScript.
• Fewer underconstrained-implicit-instantiation errors for generic calls passed as arguments to other generic calls.
• Fixed a crash when printing long types that contain non-ASCII names in error messages.
• Tuple types with a spread keep the variance of their own elements, e.g. a in [readonly a: unknown, ...T] stays read-only.
• First in [infer First, ...infer Rest] is now inferred instead of unknown.
• Fixed a deadlock in the Flow server monitor that could make all subsequent commands fail with "Out of retries, exiting!".
• Type guards with a union type now refine union members precisely, e.g. a falsy-value guard on string | number refines to "" | 0.
• Type guards now drop union members that are instances of classes unrelated to the guarded class, e.g. x: A | B refined by x is A is now A rather than A | (A & B).
• The negative branch of a type guard now refines away generics that satisfy the guard, e.g. x: T | number refined by !isT(x) with isT: (x: unknown) => x is T is now number.
• In the negative branch of a type guard, any now stays any instead of becoming empty, and boolean is refined to the literal the guard does not cover.
• Correct stdin behavior for flow check-contents and flow autocomplete.
• Error messages print code that contains backticks, such as template literals, as markdown code spans.
• Instances of TypeScript-declared generics, such as React.MouseEvent<T>, now compare type parameters without a variance annotation covariantly, as type applications already did.
• Type guards now drop tuple members of a different length and array members with disjoint element types.
• Type guards now drop object and interface union members whose properties conflict with the guard type.
• x instanceof C no longer refines a value of an intersection type to empty.
• Linux release builds use jemalloc, which keeps the memory of long-running servers from growing after large rechecks.
• Fixed a race that could leave a file unmerged after a recheck, which made the server recheck forever while an open file imported it.

Misc:

• Print rest arrays as array types in errors.
• Print receiver and merged declaration types in errors.
• Simplify missing this annotation diagnostics.
• Print both types in inheritance errors.
• Identify missing parameter annotations directly at the parameter location.
• Remove empty filename metadata from the Try Flow AST display.
• Add a default-on option to hide locations in the Try Flow AST pane.
• Show node types first in the Try Flow AST pane.
• Add a default-on option to hide null and undefined fields in the Try Flow AST pane.
• Move Try Flow's JSON error output into an Errors format pill bar.
• Add JSON AST output to Try Flow.

Parser:

• Accept explicit semicolons on shorthand ambient modules.
• Accept keyword members in qualified type names and import types.
• Allow repeated inline interface exports in TypeScript files.
• Fixed flow-parser taking minutes to parse JS files over ~1 MB.
• Support destructured rest parameters in function type signatures.
• Support import attributes on re-export declarations.
• Preserve TypeScript accessibility modifiers on bodyless class method signatures.
• Preserve computed type-literal keys as properties in TypeScript source files.
• Allow newline-separated type members in TypeScript source files.
• Use "in out" for the ESTree variance kind of in out type parameters.
• Parse postfix non-null assertions with type syntax enabled, preserve ASI, and emit TypeScript-compatible NonNullExpression nodes.
• Respect JavaScript line terminators and Unicode whitespace when recovering source locations.
• Parse default interface exports and report unsupported syntax during type checking.
• Correct inferred ESTree source type for default interfaces, inline type specifiers, and exported import aliases.
• Exclude default interface exports and all-type specifier lists from inferred Babel module source type.
• Support top-level await for explicit modules and .mjs/.mts files in flow-parser.
• Always enable type parsing for TypeScript filenames.
• Reject type syntax in untyped parser mode and report precise diagnostics.
• Fix declaration method signature locations to include the complete signature.
• Include the const keyword in const enum declaration locations.

IDE:

• Hover and error messages no longer print redundant parentheses around function types in maybe types, e.g. ?() => void instead of ?(() => void).

Library Definitions:

• Align well-known symbol members (Symbol.toStringTag, Symbol.match, Symbol.toPrimitive, ...) with TypeScript.
• Map and WeakMap set, and Set and WeakSet add, now return this.
• RegExp flags/sticky/unicode and Number.EPSILON/MAX_SAFE_INTEGER/MIN_SAFE_INTEGER are now read-only; String.prototype.normalize accepts the normalization form literals and the legacy HTML methods (bold, link, ...) are declared.
• Array.prototype.copyWithin returns this, so subclasses of Array keep their type.
• Array.prototype.sort returns this, so subclasses of Array keep their type.
• Math constants (Math.PI, Math.E, ...) are read-only.
• Number.MAX_VALUE, MIN_VALUE, NaN, NEGATIVE_INFINITY and POSITIVE_INFINITY are read-only.
• String#split requires a separator, String#replace has separate overloads for a string and a function replacement, and String#length is read-only, as in TypeScript.
• Use TypeScript's ES2018 async iterator and iterable definitions.
• Use TypeScript's ES2018 async generator definitions.
• Use TypeScript's ES2019 array definitions while preserving Flow's existing flatMap union order and inference.
• Use TypeScript's ES2020 promise definitions, including mutable settled results.
• Use TypeScript's ES2016 array includes definitions while keeping Flow's element refinement.
• Use TypeScript's ES2015 generator definitions. Generator.prototype.return now requires a value of the generator's return type.
• Add React.CSSProperties, typed by csstype.
• Use TypeScript's ES2015 promise definitions, adapted to Flow's inference.
• Use TypeScript's ESNext set method definitions, including Set.prototype.union.
• Use TypeScript's ES2015 Map, Set, WeakMap and WeakSet definitions. size is now read-only.
• done is now optional in the non-final branch of IteratorResult, as in TypeScript.
• Use TypeScript's ES2015 iterable definitions. ArrayIterator, MapIterator, SetIterator and StringIterator are now interfaces, and StringIterator takes a type argument.
• Add React's DOM types from @types/react, such as React.MouseEvent, React.HTMLAttributes and React.JSX.IntrinsicElements.
• React.ComponentProps of a string that is...

v0.337.0

Likely to cause new Flow errors:

• Function.prototype.bind is now typed with TypeScript's CallableFunction.bind overloads, so errors in bound arguments are reported at bind, and a bound function's remaining parameters are a tuple type.
• Function and Object no longer mean any and no longer trigger unclear-type errors. They are now interpreted as the prototypes of functions and objects. Code that assumed they meant any may get new errors.
• Internal type Function$Prototype$Bind is removed.
• Class method bind now uses the declared Function.prototype.bind overloads. Bound methods may now report tuple arity errors when used as callbacks that ignore extra arguments or omit nullable parameters, because their inferred parameters are represented as tuple rest parameters.

New Features:

• Support assertion functions. If you relied on Flow's special handling of invariant, give it an asserts signature.
• Execute runtime Flow syntax in CommonJS and ES modules with original stack locations through flow-node.
• Support assertion narrowing through this and super, including receiver assertions.
• Readonly now supports Array and ReadonlyArray types.

Notable bug fixes:

• Honor opaque upper bounds in template interpolation and nullable subtyping.
• Preserve nominal opaque inference when checking nullable targets.
• Fixed spurious Recursion limit exceeded errors when checking conditions, match and other expressions whose type is a trivially recursive generic type alias.
• Prevent unsound assertion narrowing when annotations refer to shadowed local declarations.
• Qualified type references through a union of namespaces (e.g. (cond ? A : B).T via a local) now look up the type in each namespace instead of reporting it as missing.
• Prevent saved-state startup and incremental match checking from crashing when a recheck is canceled.

Misc:

• Document assertion functions.

Parser:

• Add missing AST fields and node types to the Flow parser output. implicitDeclare is now omitted unless true.
• Emit TypeParameter.bound as a bare type node or null in ESTree output. Babel output retains the TypeAnnotation wrapper, and the TypeScript-to-Flow API translator generates extends bounds.
• ESTree enum declarations now use a unified EnumBody node with string-or-null explicitType. Babel output retains its existing enum body types.
• Parse typed function overload signatures in TypeScript implementation files.
• Parse destructured parameters in TypeScript function types outside declaration files.
• Parse optional class method signatures in JavaScript and TypeScript files.
• Parse optional TypeScript constructor parameter properties.
• Parse namespace export aliases and ambient exported members in TypeScript definitions.
• Fix parsing of class modifiers before synchronous generator methods.
• Support readonly T[] in unlabeled tuple elements and require parentheses when optional.

Library Definitions:

• Add Intl.Segmenter and Intl.supportedValuesOf.
• Add Intl.NumberFormat.prototype.formatRange and formatRangeToParts, and the ES2023 NumberFormat options; format accepts bigint and numeric strings.
• String.prototype.localeCompare, toLocaleLowerCase and toLocaleUpperCase accept Intl.LocalesArgument; matchAll with a RegExp returns RegExpExecArray matches.
• Float16Array.prototype.toLocaleString takes Intl.NumberFormatOptions.
• Add ES2024 ArrayBuffer resizing, transfer, and constructor options.
• Align Reflect APIs with TypeScript property, descriptor, key, and constructor definitions.
• Reuse TypeScript's Float16Array declarations while preserving Flow covariance and callback compatibility.
• Align Proxy APIs with TypeScript and check array elements against numeric interface indexers.

v0.336.0

Likely to cause new Flow errors:

• Calls such as Array.from can now infer narrower literal types, requiring explicit annotations where broader types are needed. Unconstrained class type arguments now use their bounds instead of empty.
• Flow now checks ref compatibility for JSX tags typed as string, exposing refs that assume a more specific element type.
• Explicit this parameters on methods in library definitions no longer need to accept the instance type.
• Generic rest-parameter inference can now expose previously hidden parameter-type errors. For example, (input: string, ...args: Array<unknown>) => void no longer satisfies (...args: TArgs) => R when TArgs is inferred as Array<unknown> (Try Flow example).
• Rest destructuring of exact tuples, e.g. const [first, ...rest] = t, now excludes preceding elements' types from rest.
• Variance annotations are now checked through tuple type spreads, so type T<out X> = [string, ...[X]] is an error.

New Features:

• Support tuple types with trailing rest elements, e.g. [string, ...Array<number>]. Array literals ending in array spreads and arguments collected by rest parameters can flow to these types.

Notable bug fixes:

• Declaration merging for interfaces and declare class now preserves variance annotations.
• Returns inside functions nested in a type guard function are no longer checked against the enclosing type guard.
• References inside declare namespace blocks now see members merged across blocks and library files.
• Functions and constructors are now accepted as builtin Function and Object values.
• Fixed spurious indexed-access errors involving recursive class type-argument bounds.
• Type arguments used in later tuple spreads, e.g. B in [...A, ...B], are now inferred instead of left underconstrained.
• Conditional types using infer on rest parameters now preserve the matched function's optional parameters and rest element in the inferred tuple, e.g. [number, ...Array<boolean>]. Matching functions with fewer parameters than the pattern and no rest parameter no longer produces underconstrained errors.

Misc:

• flow-upgrade now uses Flow parser packages and requires Node 18 and Prettier 3.

Parser:

• Correct JavaScript source columns after characters represented by surrogate pairs.

IDE:

• Hover Go to links no longer include the hovered declaration itself.

Library Definitions:

• Intl is now a builtin namespace with TypeScript-compatible definitions through ES2020, including Collator, NumberFormat, DateTimeFormat, PluralRules, RelativeTimeFormat, Locale, DisplayNames, getCanonicalLocales, and formatToParts for dates and numbers.
• The legacy Intl$CollatorOptions, Intl$DateTimeFormatOptions, and Intl$NumberFormatOptions aliases now match TypeScript. Intl.DateTimeFormatPartTypes includes 'unknown', and locale-aware Date methods accept Intl.LocalesArgument and Intl.DateTimeFormatOptions.
• The global LocalesArgument type is now Intl.LocalesArgument. Library declarations such as flow-typed's declare var Intl hide the builtin namespace and must be removed.
• React.ElementConfig is removed; use React.ComponentProps instead.
• Typed arrays gain findLast, findLastIndex, toReversed, toSorted, and with, and their constructors can be called without arguments. For types other than Float16Array, findLast narrows its result with a type-guard callback. Callbacks to findLast and findLastIndex can no longer annotate this.
• Float16Array now matches TypeScript's declaration, including its from overloads. length, byteLength, and byteOffset are read-only, find and findIndex predicates must return boolean, and callbacks can no longer annotate this.
• Add RegExp indices and flags, well-formed string methods, bigint Atomics, Error.isError, Promise.try, Atomics.pause, and additional ArrayBuffer and Date constructor signatures.
• Add TypeScript signatures for Array.fromAsync.
• AggregateError.prototype.errors is now typed any[], matching TypeScript.
• Decorator definitions now match TypeScript: ClassDecorator accepts classes, decorator context and metadata types are available, and Function includes [Symbol.metadata].
• Add DisposableStack, AsyncDisposableStack, SuppressedError, Disposable, and AsyncDisposable. AsyncGenerator now extends AsyncIteratorObject and includes [Symbol.asyncDispose].

v0.335.0

Breaking Changes:

• Removed the legacy builtin_lib=flowlib and builtin_lib=flowlib-with-lib-dom-d-ts options.
• Removed the Flow config options exact_by_default, experimental.always_generalize_jsx, experimental.fast_symlink_resolution, experimental.importable_global_libdefs, experimental.interface_dictionary_typing_fix, experimental.new_this_typing, experimental.opaque_type_new_bound_syntax, saved_state.allow_reinit, saved_state.direct_serialization, and saved_state.reinit_on_lib_change. These options had been enabled by default and only accepted true.

Likely to cause new Flow errors:

• Symbol and Promise now use TypeScript-style instance and constructor interfaces. Code relying on their previous class typing may see different errors.
• Some JSX expressions that were previously considered underconstrained now infer unknown.
• Fixed type argument inference that incorrectly used a bound or unknown. More underconstrained errors may appear.
• String.prototype.replaceAll now has separate overloads for a replacement string and a replacer function, matching TypeScript. Passing a union of these types is an error.
• RegExpMatchArray.groups is now optional and cannot be null, and RegExp.prototype.dotAll is read-only, matching TypeScript.
• new $ReadOnlyArray now causes an error.
• Symbol() now only accepts a string | number description, and Symbol.keyFor returns string | void, matching TypeScript.
• Deprecated syntax now consistently causes errors, including in library definition files.
• Type arguments of a polymorphic value flowing to a non-polymorphic type are now solved, which can surface previously hidden errors.
• React.PropsOf now uses React.ComponentProps to find component props. Additional errors may occur.

New Features:

• Support assertion functions exported through CommonJS, assertion calls in return position, and assertion calls in arrow expression bodies.
• The undefined type can be used in library definitions, where it means void.
• flow-remove-types supports abstract classes, abstract members, override modifiers, satisfies, optional class fields, and additional type-only imports and exports.
• Template literals infer precise types in const declarations and annotated initializers.

Notable bug fixes:

• as const on template literals infers template literal types like TypeScript.
• Implicit instantiation can infer a type argument through React.ComponentProps and other conditional types with infer.
• Interfaces with construct signatures no longer print a spurious this parameter in error messages and hovers.
• this-typed methods inherited from builtins such as typed arrays now return the subclass.
• Exported declare class declarations extending builtins such as Error, Map, Set, or Date now inherit the builtin's statics and constructor.
• Arguments of new X(...) are now contextually typed when X is annotated with the typeof of a constructor interface value.
• typeof x === 'function' now refines builtin constructors such as Map and Symbol, and interfaces with call signatures, as functions.
• User-defined assertion conditions check member access in test context.
• Recognize ESM default imports of CommonJS assertion functions and callable objects with asserts call signatures.
• instanceof and declare class ... extends now use the prototype of a constructor interface that has no construct signature, instead of narrowing to empty or reporting "not inheritable".
• Fixed a memory leak in type signature parsing that inflated server memory on large codebases.
• Fixed checking interfaces that extend classes, including object values and multiple generic class supers.

Misc:

• React.ElementConfig is deprecated in this version and will be removed in the next version. Use React.ComponentProps instead.
• The unnecessary-invariant lint is renamed to unnecessary-assertion; the old name is still accepted.
• Constructor errors and variance check errors now print types instead of descriptions such as "object type", "module", or "exports".
• Unary arithmetic on a maybe-typed operand reports null and void separately instead of "null or undefined".
• Record type utility errors name the utility (Partial, Required, or mapped type).

Parser:

• Store abstract property annotations in typeAnnotation and emit value: null.

IDE:

• Hover on flow.org/try now shows types framed as declarations, matching the VS Code extension.
• Autocomplete and go-to-definition on instances of classes extending builtins such as Map, Set, Error, and Date now show the builtin's instance members.
• JSX attribute autocomplete on a union or intersection of components offers the props of the components the element is checked against.
• Go-to-definition on a JSX attribute of a union or intersection of components uses the props of the components the element is checked against.
• Find-references on a JSX attribute of an intersection of components finds the attributes passed to the branch the element is checked against.
• Qualify React.ComponentProps in generated type annotations.

Library Definitions:

• React.ComponentProps<'div'> now includes the optional ref prop.
• Added Promise.withResolvers.
• The type parameters of Map.groupBy are now ordered <K, T>, matching TypeScript.
• RegExp.prototype[Symbol.matchAll] is now a method and RegExpStringIterator an interface, matching TypeScript.

v0.334.0

Likely to cause new Flow errors:

• Improved generic type inference with conditional types. Some places might infer a different type and cause new errors.
• Number, String, RegExp, Date, native errors, maps, and sets now use TypeScript-style instance and constructor interfaces, while Math, JSON, and Atomics use named interfaces. Existing code relying on their previous class or object typing may produce new errors.

New Features:

• Allow object type spreads over interfaces and inexact object types.

Notable bug fixes:

• Allow classes to extend generic construct-signature interfaces.
• Generic values re-exported through annotated CommonJS getters retain their polymorphic types.
• Preserve record lowering source locations.
• Preserve match lowering source locations.

Parser:

• Support semicolonless abstract method declarations.

v0.333.0

Likely to cause new Flow errors:

• Imports of react module will always resolve to Flow's builtin react type definitions. If you use a custom react typing module, you might see errors.

Notable bug fixes:

• Preserve Flow WASM linker settings, including memory growth.
• Correct parser ranges for unique symbol types and signed mapped-property variance.

Parser:

• Rename the ImportType ESTree field from argument to source.
• Preserve computed object type property keys in parser output.

Library Definitions:

• Flow will now include DOM typing from TypeScript's lib.dom.d.ts by default. If your project is not supposed to run in the browser, you can switch back to the old behavior with builtin_lib=core-only in flowconfig's [options] section.
• Flow's Iterable and ArrayLike type now align with those from TypeScript. You might see errors when your existing code assumes the old typing.

v0.332.0

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)

v0.331.0

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 .

v0.330.0

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)

v0.329.0

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.