“X”类型的参数不能分配给“Y”类型的参数

Question

I have created those types:

export type Optional<T> = T | null;

and I have this function

updateBook( book: Array<Optional<Hostel>>)

that I call with

 let book: Hostel | null [] = [null];
 updateBook(book)

but I have this error

 - error TS2345: Argument of type 'Hostel | null[]' is not assignable to parameter of type 'Optional<Hostel>[]

Answer 1

Hostel | null [] Hostel | null [] means a union between Hostel and null[] . The [] has a higher precedence than the union operator ( | ).

You probably wanted to say an array of Hostel | null Hostel | null which can be written as (Hostel | null)[] or, more readably IMO Array<Hostel | null> Array<Hostel | null> .