What is the solution for resolving the error message, "The type 'string | boolean' cannot be assigned to type 'never'. The type 'string' cannot be assigned to type 'never' "?

Question

What is the solution for resolving the error message, "The type 'string | boolean' cannot be assigned to type 'never'. The type 'string' cannot be assigned to type 'never' "?

interface DataStructure {
    name: string;
    url: string;
    date: string;
    popular: boolean;
}

const itemData: DataStructure = {
    name: "Item 1",
    url: "item-1",
    date: "2012",
    popular: true,
};

function getItemByURL(keys: Array<keyof DataStructure>) {
    const itemsList = <DataStructure>{};
    // Ensure only the necessary data is shown
    keys.forEach((key) => {
        if (key === "popular") {
            itemsList[key] = itemData.popular;
        }
        if (typeof itemData[key] !== "undefined") {
            itemsList[key] = itemData[key]; 
        }
    });
    return itemsList;
}

I am encountering an issue with "itemsList[key]". I have tried solutions from similar problems, but none have worked for me. Can someone point out what I am missing here? Error: Type 'string | boolean' is not assignable to type 'never'. Type 'string' is not assignable to type 'never'

typescript

Answer 1

Answer №1

The problem arises because the items object is empty, so items[field] will never exist.

By switching to a reduce method, you can both resolve the error (since you are not attempting to access a non-existent value) and tidy up your function.

function getCourseBySlug(fields: Array<keyof IData>): IData {
    return fields.reduce((acc: IData, field: keyof IData) => {
        if (typeof product[field] !== "undefined") {
            return { ...acc, [field]: product[field] };
        } else {
            return acc;
        }
    }, {
        title: "",
        slug: "",
        published_at: "",
        isPopular: false,
    });
}

--edit--

If you want the returned object to be a subset of the original, as mentioned in the comments, you can use Partial for the return and accumulator types.

function getCourseBySlug(fields: Array<keyof IData>): Partial<IData> {
    return fields.reduce((acc: Partial<IData>, field: keyof IData) => {
        if (typeof product[field] !== "undefined") {
            return { ...acc, [field]: product[field] };
        }
    }, {});
}

Answer 2

The problem arises because the items object is empty, so items[field] will never exist.

By switching to a reduce method, you can both resolve the error (since you are not attempting to access a non-existent value) and tidy up your function.

function getCourseBySlug(fields: Array<keyof IData>): IData {
    return fields.reduce((acc: IData, field: keyof IData) => {
        if (typeof product[field] !== "undefined") {
            return { ...acc, [field]: product[field] };
        } else {
            return acc;
        }
    }, {
        title: "",
        slug: "",
        published_at: "",
        isPopular: false,
    });
}

--edit--

If you want the returned object to be a subset of the original, as mentioned in the comments, you can use Partial for the return and accumulator types.

function getCourseBySlug(fields: Array<keyof IData>): Partial<IData> {
    return fields.reduce((acc: Partial<IData>, field: keyof IData) => {
        if (typeof product[field] !== "undefined") {
            return { ...acc, [field]: product[field] };
        }
    }, {});
}

Answer 3

Answer №2

Here are two distinct issues:

The items you are using is not a valid IData object. It is missing some of the required IData properties.
In your loop, TypeScript is unable to determine the specific type of field (it only knows that it's one of the keys of IData), which leads to uncertainty in whether items[field] should be a string or a boolean. As a result, assigning a value to it becomes problematic. Even though you may be aware that items[field] and product[field] will have the same type, TypeScript does not have that knowledge.

The solution to the first issue is simple: Define it as a partial IData:

const items: Partial<IData> = {};

You have a few options to address the second issue:

Enhance the function's type safety using a helper function.

function copyProp<ObjType extends object, KeyType extends keyof ObjType>(obj: ObjType, key: KeyType, value: ObjType[KeyType]) {
    obj[key] = value;
}
function getCourseBySlug(fields: Array<keyof IData>) {
    const items: Partial<IData> = {};
    for (const field of fields) {
        copyProp(items, field, product[field]);
    }
    return items;
}

This method ensures type compatibility between the value passed in (product[field]) and the target object's field through ObjType[KeyType].

Ignore the TypeScript error, although this is rarely recommended. If you are certain that the error is inconsequential, you can choose to ignore it.

function getCourseBySlug(fields: Array<keyof IData>) {
    const items: Partial<IData> = {};
    for (const field of fields) {
        // @ts-ignore
        items[field] = product[field];
    }
    return items;
}

Work around the issue by utilizing Object.fromEntries:

function getCourseBySlug(fields: Array<keyof IData>) {
    const items: Partial<IData> = Object.fromEntries(fields.map(field => [field, product[field]]));
    return items;
}

The first approach is fully typesafe, while the second and third methods are somewhat imperfect but localized solutions. In the context of the getCourseBySlug function, you can ensure that items[field] and product[field] have the same type.

It is important to note that I have omitted the

typeof product[field] !== "undefined"

check in all these solutions, as the type of fields guarantees that field is a valid key for product, and none of the IData properties are optional.

Answer 4

Here are two distinct issues:

The items you are using is not a valid IData object. It is missing some of the required IData properties.
In your loop, TypeScript is unable to determine the specific type of field (it only knows that it's one of the keys of IData), which leads to uncertainty in whether items[field] should be a string or a boolean. As a result, assigning a value to it becomes problematic. Even though you may be aware that items[field] and product[field] will have the same type, TypeScript does not have that knowledge.

The solution to the first issue is simple: Define it as a partial IData:

const items: Partial<IData> = {};

You have a few options to address the second issue:

Enhance the function's type safety using a helper function.

function copyProp<ObjType extends object, KeyType extends keyof ObjType>(obj: ObjType, key: KeyType, value: ObjType[KeyType]) {
    obj[key] = value;
}
function getCourseBySlug(fields: Array<keyof IData>) {
    const items: Partial<IData> = {};
    for (const field of fields) {
        copyProp(items, field, product[field]);
    }
    return items;
}

This method ensures type compatibility between the value passed in (product[field]) and the target object's field through ObjType[KeyType].

Ignore the TypeScript error, although this is rarely recommended. If you are certain that the error is inconsequential, you can choose to ignore it.

function getCourseBySlug(fields: Array<keyof IData>) {
    const items: Partial<IData> = {};
    for (const field of fields) {
        // @ts-ignore
        items[field] = product[field];
    }
    return items;
}

Work around the issue by utilizing Object.fromEntries:

function getCourseBySlug(fields: Array<keyof IData>) {
    const items: Partial<IData> = Object.fromEntries(fields.map(field => [field, product[field]]));
    return items;
}

The first approach is fully typesafe, while the second and third methods are somewhat imperfect but localized solutions. In the context of the getCourseBySlug function, you can ensure that items[field] and product[field] have the same type.

It is important to note that I have omitted the

typeof product[field] !== "undefined"

check in all these solutions, as the type of fields guarantees that field is a valid key for product, and none of the IData properties are optional.

What is the solution for resolving the error message, "The type 'string | boolean' cannot be assigned to type 'never'. The type 'string' cannot be assigned to type 'never' "?

Answer №1

Answer №2

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