It seems that the Flow types in Preact packages are wrong and misleading. I've set up a very simple example - a component rendering another component, which requires some props. The component is rendered without passing any props in, with the expectation that Flow will error out.
In the React example, Flow correctly detects the problem:
➜ react git:(master) ✗ npx flow
Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ index.js:25:13
Cannot create MyComponentWithProps element because:
• property a is missing in props [1] but exists in Props [2].
• property b is missing in props [1] but exists in Props [2].
• property c is missing in props [1] but exists in Props [2].
[2] 12│ class MyComponentWithProps extends React.Component<Props> {
:
22│
23│ class App extends React.Component<*> {
24│ render() {
[1] 25│ return <MyComponentWithProps />
26│ }
27│ }
28│
Found 3 errors
In the Preact example, however, Flow is happy:
➜ preact git:(master) ✗ npx flow
No errors!
If you compare Preact's Flow types with its
TypeScript counterpart, you'll notice that they're vastly different. preact.js.flow
doesn't expose many of the types that are available in TypeScript definitions, meaning that Flow defaults to any for these. I also don't think that
re-exporting React types as it currently set up works if I'm reading flow-typed docs correctly:
https://github.com/flow-typed/flow-typed/wiki/FAQs#how-do-i-build-type-definitions-that-depend-on-other-definitions