React/Redux reducer typescript error (Type 'undefined' is not assignable to type ISupplierState)

Question

I haven't been able to solve this issue for a long time even though I've been searching through various pages with similar problems.

I'm receiving the following error:

src/app/store/instructor/supplier/reducer.tsx
TypeScript error in src/app/store/instructor/supplier/reducer.tsx(11,7):
Type '(state: ISupplierState | undefined, action: AnyAction) => { loading: true; suppliers: Supplier[]; errors?: string | undefined; } | { suppliers: any; loading: false; errors?: string | undefined; } | { ...; } | undefined' is not assignable to type 'Reducer<ISupplierState, AnyAction>'.
  Type '{ loading: true; suppliers: Supplier[]; errors?: string | undefined; } | { suppliers: any; loading: false; errors?: string | undefined; } | { loading: false; errors: any; suppliers: Supplier[]; } | undefined' is not assignable to type 'ISupplierState'.
    Type 'undefined' is not assignable to type 'ISupplierState'.  TS2322

     9 | };
    10 | 
  > 11 | const reducer: Reducer<ISupplierState> = (state = initialState, action) => {
       |       ^
    12 |   switch (action.type) {
    13 |     case ISupplierActionTypes.ISUPPLIER_FETCH: {
    14 |       return {

Here is my reducer:

export const initialState: ISupplierState = {
  suppliers: [],
  loading: false,
  errors: undefined,
};

const reducer: Reducer<ISupplierState> = (state = initialState, action) => {
  switch (action.type) {
    case ISupplierActionTypes.ISUPPLIER_FETCH: {
      return {
        ...state,
        loading: true,
      };
    }
    case ISupplierActionTypes.ISUPPLIER_SUCCESS: {
      return {
        ...state,
        suppliers: action.payload,
        loading: false,
      };
    }
    case ISupplierActionTypes.ISUPPLIER_ERROR: {
      return {
        ...state,
        loading: false,
        errors: action.payload,
      };
    }
  }
};

export { reducer as iSupplierReducer };

Here is my types / ISupplierState:

export interface Supplier {
  name: string;
  id: number;
  payment_terms: number;
  address_id: number;
  delivery_time: number;
  bank_id: number;
  company_Type_id: number;
}

export interface ISupplier {
  suppliers: Supplier[];
}

export enum ISupplierActionTypes {
  ISUPPLIER_FETCH = '@@instructor/supplier/REQUEST',
  ISUPPLIER_SUCCESS = '@@instructor/supplier/SUCCESS',
  ISUPPLIER_ERROR = '@@instructor/supplier/ERROR',
}

export interface ISupplierState {
  readonly loading: boolean;
  readonly suppliers: Supplier[];
  readonly errors?: string;
}

I understand the error code but I am able to do a very similar approach on another reducer without any issues. So I can't see why this is causing so much problem.

I am using Redux 7.2.0 Any help would be appreciated :) Thank you in advance.

Answer 1

My apologies. I had forgot to add a default return to my switch.. So if you encounter a similiar problem, then remember to check your switch!

Adding following lines solved it.

default: {
    return state;
}