/**
 * @packageDocumentation
 * @module Number
 */
import { OperatorFunction, Subscribable } from 'rxjs';
/**
 * Returns an Observable that emits a formatted string of a number raised to an exponential power using
 * Number.toExponential
 *
 * @category Formatting
 *
 * @param exponential The exponential value to raise the number by
 *
 * @example Return a string passed numbers to the exponential of `2`
 * ```ts
 * const input = [-1, 0, 1, 2, 3.4];
 * from(input).pipe(toExponential(2)).subscribe();
 * ```
 * Output: `'-1.00e+0', '0.00e+0', '1.00e+0', '2.30e+0', '3.14e+0'`
 *
 * @returns Observable that emits a formatted string of the exponential number

 */
export declare function toExponential(exponential: Subscribable<number> | number): OperatorFunction<number, string>;