property X is read-only in CategoryType [1] but writable in object type [2] in array element.

categories.js

// @flow
export type CategoryType = {
    +id: number,
    +name: string,
    +slug: string,
    +description: string
};

// ...
const categoriesItems = state.categories.items.slice();

return {
  categories: formatCollectionForDropdown(categoriesItems)
};

// ...

selectors.js

/** @flow */
export function formatCollectionForDropdown(
    collection: Array<{
        id: number,
        slug: string,
        name: string,
        description: string
    }>
): Array<{
    key: number,
    slug: string,
    text: string,
    value: number
}> {
    return collection.map(
        (
            item: {
                id: number,
                slug: string,
                name: string
            }
        ): {
                key: number,
                slug: string,
                text: string,
                value: number
            } => {
            return {
                key: item.id,
                slug: item.slug,
                text: item.name,
                value: item.id
            };
        }
    );
}