/**
 * Converts a nested object structure into a flat object with dot notation and
 * bracket notation for arrays and special characters.
 *
 * @param obj - The nested object to flatten.
 * @param prefix - The prefix for nested keys (used internally for recursion).
 * @returns A flat object with keys in dot or square bracket notation.
 *
 * @example
 * // Complex nested structure with arrays and special characters
 * const nested = {
 *   user: { name: 'John', address: { city: 'NY' } },
 *   scores: [85, 90, 95],
 *   items: [{ id: 1, name: 'Laptop' }, { id: 2, name: 'Phone' }],
 *   'property.with.dots': true
 * };
 *
 * flattenObject(nested);
 * // {
 * //   'user.name': 'John',
 * //   'user.address.city': 'NY',
 * //   'scores[0]': 85,
 * //   'scores[1]': 90,
 * //   'scores[2]': 95,
 * //   'items[0].id': 1,
 * //   'items[0].name': 'Laptop',
 * //   'items[1].id': 2,
 * //   'items[1].name': 'Phone',
 * //   'property\\.with\\.dots': true
 * // }
 *
 * @example
 * // Simple nested object
 * flattenObject({ user: { name: 'Alice', age: 30 } });
 * // { 'user.name': 'Alice', 'user.age': 30 }
 *
 * @example
 * // Array of primitives
 * flattenObject({ numbers: [1, 2, 3] });
 * // { 'numbers[0]': 1, 'numbers[1]': 2, 'numbers[2]': 3 }
 *
 * @example
 * // Empty object
 * flattenObject({}); // {}
 *
 * @example
 * // Real-world: Flatten API response for form data
 * const apiResponse = {
 *   user: { profile: { firstName: 'John', lastName: 'Doe' } },
 *   settings: { notifications: true }
 * };
 * flattenObject(apiResponse);
 * // {
 * //   'user.profile.firstName': 'John',
 * //   'user.profile.lastName': 'Doe',
 * //   'settings.notifications': true
 * // }
 *
 * @note Array indices use square bracket notation: 'items[0].name'
 * @note Dots in property names are escaped with backslashes: 'property\\.with\\.dots'
 * @note Empty objects and arrays are preserved in the flattened structure
 * @note Useful for form data serialization and database queries.
 * @note Opposite operation can be performed with unflattenObject().
 *
 * @complexity Time: O(n), Space: O(n) - Where n is total properties including nested
 */
export declare function flattenObject(obj: Record<string, unknown>, prefix?: string): Record<string, unknown>;
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