facebook/flow v0.230.0

on GitHub

Likely to cause new Flow errors:

• We now put value exports and type exports into two different namespaces. It means that we will now consistently error on places where you try to import, re-export, or use a imported type as a value. This change also makes certain patterns that happen to work in the past no longer work, e.g.

// a.js
export type Foo = string;
// b.js
const A = require('./a');
module.exports = {...A};
// c.js
// previously allowed, no longer allowed
import type {Foo} from './b';

• We have allowed function parameters to refer to each other for a while now. However, we didn't implement the analysis correctly for exported functions, which causes inconsistent behaviors. This is now fixed e.g.

// a.js
// we incorrectly resolve x to refer to
// be external to the function params,
// which is a global in this case
export function foo(x: string, y: typeof x) {}
// b.js
import {foo} from './a';
foo("", 3); // no error before, errors now

• Previously, we allowed assignment to any globals, regardless of whether it's a type or value. Therefore, both were allowed in Array = 3; $ReadOnlyArray = 2; Now the latter is banned.
• Flow now consistently bans referring a type-only global as value
• Some bad annotations that cause value-as-type or recursive-definition errors will no longer affect which branch to take in overload resolution.

New Features:

• Flow now supports typeof with type arguments type T = typeof foo<string>. This syntax is supported in prettier since v3.1. If you previously enabled the flag typeof_with_type_arguments=true, you need to remove it.
• Flow now supports explicit type arguments on JSX. e.g. <Foo<string, _, number >propA='d' />. Note that some type arguments can be inferred using the _ syntax.
• Flow now supports the same NoInfer intrinsic that will be available in TypeScript 5.4.
• Under experimental.ts_syntax=true, Flow will
  • Automatically translate TypeScript's readonly on tuple and array types into Flow equivalent $ReadOnly<[...]> and $ReadOnlyArray<...> without error.
  • Automatically translate TypeScript's keyof into Flow equivalent $Keys without error. Note that there are behavior differences between $Keys<> and keyof right now (flow, TS), so use it with caution.
  • Automatically translate TypeScript's unknown never and undefined into Flow equivalent mixed empty and void without error.
  • Support TypeScript's variance annotation readonly in out, in out without error.

Misc:

• experimental.ts_syntax is now always on in https://flow.org/try.