export declare const toString: () => string;
export declare function isDate(val: any): val is Date;
export declare function isObject(val: any): val is Object;
export declare function isPlainObject(val: any): val is Object;
export declare function isDef(val: any): boolean;
export declare function isUnDef(val: any): boolean;
/**
 * Determine if a value is a String
 *
 * @param {*} val The value to test
 *
 * @returns {boolean} True if value is a String, otherwise false
 */
export declare const isString: (thing: any) => boolean;
export declare function extend<T, U>(to: T, from: U): T & U;
export declare function deepMerge(...objs: any[]): any;
export declare function isFormData(val: any): boolean;
export declare function isURLSearchParams(val: any): val is URLSearchParams;