facebook/flow v0.257.0

on GitHub

Likely to cause new Flow errors:

• We have improved inference for exported primitive literal values.
  • Before this change the inferred type for export const obj = {f: "foo"}; was {f: str<"foo">}, where str<"foo"> was a type that opportunistically and unsoundly behaved either as string or as the singleton type "foo".
  • Flow will now infer the type {f: string} for obj.
  • For const like exports, e.g. export const x = "foo"; Flow will export the type "foo".
  • To fix errors that arise due to this change, you can provide annotations around exported literals or use the as const modifier.
• We fixed a major unsoundness with regards to dictionary object creation. Previously, the computed property will just be ignored for string or any keys. Now, if the computed property is added to some existing objects (e.g. {foo: string, [stringTypedKey]: v}), then we will error on the property. Otherwise, the {[key]: value} will be {[typeof key}: typeof value}
• We fixed a bug that causes opaque type's underlying representation to leak beyond the file it's defined. You might see more errors as a result.
• Flow will now perform a complete function call check even when [react-rule-unsafe-mutation] errors are raised.
• Previously we incorrectly distribute-over-union for maybe or optional input types of conditional type. (e.g. (?string) extends string ? true : false is evaluated to true | false instead of false). This is now fixed, and code that depends on the bug might have new errors.
• If you have refiend a value based on typeof x === 'function', and then do typeof x === 'object', the result is now empty as that is impossible.
• Singleton types are now also considered in conditional equality checks on member expressions. (e.g. try-Flow)
• React.Config type, deprecated in v0.254, is now removed.

New Features:

• 'key' in x now refines the type of x to objects which have the property key.
  • in checks for both own and non-own properties.
  • If we find that a key does not exist in an inexact object or instance/interface, then the negation is not refined since the property may or may not exist.
  • An optional property is also considered as if it may or may not exist.
  • If a proto/super is a union, every member of the union must have it
  • If the input to the refinement is an intersection, one member of that intersection must have it
  • We don't refine arrays since it's not useful (check for .length instead), and also because Flow doesn't handle array holes.
• Flow now supports no_unchecked_indexed_access=true in flowconfig, which is equivalent to noUncheckedIndexedAccess from TS. It will add | void to every indexed access with general string key on dictionary objects or number key on arrays.
• Support for React 19's ref-as-prop model is now available via react.ref_as_prop=partial_support, and this is now the default. (To disable it, use react.ref_as_prop=disabled.) Under this mode, ref prop in jsx will be treated as a regular prop for function components, but utility types like React.ElementConfig<...> still won't include the regular ref prop yet.
• Flow now allows using union of strings as the type of computed property keys.
• You can now destructure object properties using number literal keys (which are int-like).

Notable bug fixes:

• Refining type guards with opaque types will now refine the general type (example of code that used to fail before, but now passes: try-Flow)
• Allow more flexibility in checks involving super type of opaque type (e.g. try-Flow)
• Fixed a bug when using opaque types in type guards in files other than the one where the type guard function was defined.
• We now allow a union of generic or opaque string typed values to be used as a key of computed property, if it's bounded by a string type.

Library Definitions:

• Add libdef types for getNotifications and showNotification API for ServiceWorkerRegistration