simple validation hook

useValidationHook.ts

export function useValidationHook(defaultItemState: Partial<KeywordAdItem>) {
  // Allow partial a partial state to be used as the default/inital render.
  let defaultState = {...DEFAULT_STATE};
  if (defaultItemState) {
    Object.assign(defaultState.item, defaultItemState);
  }

  return useReducer((state: State, action: Action) => {
    const { field, value } = action;
    // Create a shallow copy of the old data and add/repace the new value.
    const newState = { ...state };

    // Create a new shallow copy of the item object.
    // Use assign to set the field:value pair.
    newState.item = Object.assign({}, state.item, {
      [field]: value,
    });

    // To be valid:
    // 1. All the values must be defined.
    newState.isValid = Object.values(newState.item).every(isDefined);

    return newState;
  }, defaultState);
}