Add an optional `badge` field to the NavItem type in resources/js/types/index.d.ts.
Find the NavItem interface and add this line inside it:

    badge?: string | number;

If your NavItem looks like:
    export interface NavItem {
        title: string;
        href: string;
        icon?: ...;
        isActive?: boolean;
    }
add `badge?: string | number;` as another optional field.