/**
 * Adds unit to a value.
 *
 * @example
 *
 * ```ts
 * expect(addUnit()).toBe('')
 * expect(addUnit('100px')).toBe('100px')
 * expect(addUnit(100)).toBe('100px')
 * expect(addUnit(100, 'em')).toBe('100em')
 * ```
 */
export declare const addUnit: (value?: string | number | undefined, unit?: string) => string;