It’s simply a TypeScript error. Your syntax is problematic: const unblockRef = useRef<() => void | null>(null); This is interpreted as a function that returns either void or null. So, you need to make sure it’s a function or null: const unblockRef = useRef<(() => void) | null>(null);
This is Flowtype’s Exact Object type syntax.
I would write this as type StationType = { code: String, label: String, } function StationDetail({ station } : {station : StationType}) => { const { code, label, } = station; It’s necessary to declare the type of the object parameter that the function accepts.
What you’re looking for is the intersection type. According to the documentation: An intersection type requires a value to be all of the input types. Syntax: Intersection: < type 1 > & < type 2 > … & < type n > The intersection type is intended to extend an existing type and add additional … Read more
Basically bar: ?string accepts a string, null or void: foo(“test”); foo(null); foo() While bar?: string Accepts only a string or void: foo(“test”); foo(); As passing null instead of a string is somewhat senseless, theres no real life difference between them.
Flow doesn’t do direct casting from one type to another, but you can do something like const bar: string = (foo: any); so you cast foo to an any, because any accepts any type of value as an input. Then because the any type also allows you to read all possible types from it, you … Read more
The errors you are seeing are a combination of a bug in your code and a bug in Flow. Bug in your code Let’s start by fixing your bug. In the third else statement, you assign the wrong value to } else { name=”someone”; age = args[1]; // <– Should be index 0 } Changing … Read more
You need to define a type for the state property in order to use it. class ComponentA extends Component { state: { isExpanded: Boolean }; constructor(props) { super(props); this.state = { isExpanded: false }; } }
You need to define a type for the state property in order to use it. class ComponentA extends Component { state: { isExpanded: Boolean }; constructor(props) { super(props); this.state = { isExpanded: false }; } }
The + symbol in front of the property means the property is read-only Reference: https://flow.org/en/docs/types/interfaces/#toc-interface-property-variance-read-only-and-write-only