export declare const isExternalUrl: (url: string) => boolean;
export declare const capitalizeFirstLetter: (str: string) => string;
type ClassPart = string | undefined | null | false | ClassPart[];
/**
 * Joins class names, skipping falsy values. Arrays are flattened. Duplicate tokens
 * (including across arguments) are removed; first occurrence wins.
 * Returns `undefined` when there are no tokens (so `className={cx(...)}` omits the attribute).
 *
 * @example
 * cx('govuk-input', className, isWide && 'govuk-!-width-full')
 * cx(['govuk-fieldset', className], error && 'govuk-fieldset--error')
 */
export declare function cx(...parts: ClassPart[]): string | undefined;
export {};