Get free port via Node without packages/dependencies

getFreePort.ts

import {createServer, AddressInfo} from "net";

/**
 * Utility function to return a free port.
 * 
 * Uses dummy server that listens to port 0 to let the OS assign the server a 
 * port.
 * Then the server will be closed and the now open port will be returned.
 * 
 * @return by OS assigned, free port
 */
export default async function getFreePort(): Promise<number> {
  return new Promise(resolve => {
    
    // dummy server to have the port assigned to
    let dummyServer = createServer();
    
    // listening to port 0 will let the OS assign a port
    dummyServer.listen(0, () => {
      
      let {port} = dummyServer.address() as AddressInfo;
      dummyServer.close(() => {
        // after server is closed, the port should be free
        resolve(port);
      })
      
    });
  });
}