import type * as http from 'node:http';
import type { Plugin } from '../types.js';
/**
 * Helper function to define a http-proxy-middleware plugin
 * @see proxyServer {@link ProxyServer} - proxy server instance to which the plugin is being applied
 * @see options {@link Options} - options object passed to `createProxyMiddleware`
 *
 * @example defining a plugin
 * ```js
 * export const myPlugin = definePlugin((proxyServer, options) => {
 *   // plugin implementation
 * });
 * ```
 *
 * @example using a plugin
 * ```js
 * createProxyMiddleware({
 *   target: 'http://example.com',
 *   plugins: [myPlugin],
 * });
 * ```
 *
 * @since 4.1.0
 */
export declare function definePlugin<TReq extends http.IncomingMessage = http.IncomingMessage, TRes extends http.ServerResponse = http.ServerResponse>(fn: Plugin<TReq, TRes>): Plugin<TReq, TRes>;