How to fix error TS2345 in functional component React?-CodePudding

I have a small React Typescript application. I am running a filtering function which is executed when some buttons are clicked or a value is entered into an input.

I have interfaces everywhere. I also passed the interface to the state, or the fact that I will have an empty array. But I keep getting an error with the following text:

"TS2345: Argument of type 'TodoItemInterface[] | undefined' is not assignable to parameter of type 'TodoItemInterface[]'. Type 'undefined' is not assignable to type 'TodoItemInterface[]'."

I can’t find how and where to pass that I definitely won’t have undefined or fix this error in a different way. Please help. Below I send the code of the function in useEffect and functions with a state that are related to it.

error is thrown here in filterItems.

useEffect(() => {
    const newArray = searchInputFilter(filterItems(todoListStateArray, infoSearchObject.filter), textForSearch);

    setVisibleTodoListItems(newArray);


}, [infoSearchObject, todoListStateArray, textForSearch])

Another code

   const filterItems = (arr: TodoItemInterface[], filter: string) => {
        if (filter === 'all') {
            return arr;
        } else if (filter === 'active') {
            return arr.filter((item) => (!item.done));
        } else if (filter === 'done') {
            return arr.filter((item) => item.done);
        }

    };

    const searchInputFilter = (arr: TodoItemInterface[], search: string) => {
        if (search.length === 0) {
            return arr;
        }

        return arr.filter((item) => {
            return item.subtitle.toLowerCase().includes(search.toLowerCase());
        });
    }

    const [infoSearchObject, setInfoSearchObject] = useState({inWork: '', done: '', filter: 'all'});
    const [todoListStateArray, setTodoListStateArray] = useState<TodoItemInterface[] | []>(todoListDefault);
    const [visibleTodoListItems, setVisibleTodoListItems] = useState<TodoItemInterface[] | []>(todoListDefault);
    const [textForSearch, setTextForSearch] = useState('');

CodePudding user response：

That is actually one of the benefits in using typescript; It’s actually telling you there is the possibility that your filterItems can possibly return undefined, and that happens when neither of your conditions are true.

As a solution, you should add a return value when none of the conditions are met:

const filterItems = (arr: TodoItemInterface[], filter: string): TodoItemInterface[] => {
        if (filter === 'all') {
            return arr;
        } else if (filter === 'active') {
            return arr.filter((item) => (!item.done));
        } else if (filter === 'done') {
            return arr.filter((item) => item.done);
        }
        return arr;
    };

To note that in the example I also added a return type to your function; That is pretty important to ensure consistency in what get actually returned from it.