I have a type guard function for type User. How do I extract the type, that this function "guards"? Using ReturnType<typeof isUser> obviously does not work since the return type of the function is boolean, not User. type User = { username: string } function isUser(value: unknown): value is User { return value !== null… Read More Typescript get type, that type guard function checks

Need to type guard but instanceof doesn’t work with TypeScript types: type Letter = ‘A’ | ‘B’; const isLetter = (c: any): c is Letter => c instanceof Letter; // Error: ‘Letter’ only refers to a type, but is being used as a value here. // Expected usage: Filter via type guard. isLetter(‘a’); // Should… Read More How to type guard TypeScript type if instanceof only supports classes?

I am working with some autogenerated types from graphQL-data and need a way to make a typeguard against null, undefined and null[] to narrow types. So far the I got this: export default function validData<T>( condition: T ): asserts condition is Exclude<T, null | undefined | null[]> { if ( condition === null || condition… Read More Typeguard against null[]