What is the correct way to type number|null?

Question

typescript

I'm trying to write:

export interface ApplicationInit {
  initialPayment: number|null;
}
const initialState: ApplicationInit = {
  initialPayment: null,
};

Gives an error:

How can I fix it?

Answer 1

An interface in TypeScript can have multiple declarations in code, and TypeScript will combine them all into one generic type. Judging by the error with your ApplicationInit, this is exactly what happened, in addition to the initialPayment field, other fields are declared in another declaration, which is why it requires them.