/**
 * Converts a FormData object to a plain JavaScript object.
 * Useful for serializing form data for API requests or storage.
 *
 * @param formData - The FormData instance to convert
 * @returns A plain object with form field names as keys and their values
 * @throws {Error} If the parameter is not a FormData instance
 *
 * @example
 * ```typescript
 * const form = document.querySelector('form');
 * const formData = new FormData(form);
 * const json = formDataToJSON(formData);
 * // json might be: { username: 'john', email: 'john@example.com' }
 * ```
 */
export declare function formDataToJSON(formData: FormData): Record<string, any>;
/**
 * Converts a plain JavaScript object to a FormData instance.
 * Useful for preparing data for multipart/form-data submissions.
 *
 * @param json - The plain object to convert to FormData
 * @returns A FormData instance containing all properties from the input object
 * @throws {Error} If the parameter is not a valid object (null or non-object)
 *
 * @example
 * ```typescript
 * const data = { username: 'john', email: 'john@example.com' };
 * const formData = JSONToFormData(data);
 * // formData can now be used with fetch() for multipart uploads
 * ```
 */
export declare function JSONToFormData(json: Record<string, any>): FormData;
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