promisify a NodeJS http createServer for testing

1 - usage.md

# create a test server with canned responses
const server: TestServer = new TestServer(new Response(500, payload), new Response(200, payload))

# wait for it to start
const url = await server.ready()

# use it
fetch(url).then(async res => console.log(await res.json()), console.error)

TestServer.ts

import http, { Server } from 'http'

export class Response {
  constructor (
    public readonly code: number,
    public readonly payload: any
  ) {}
}

export class TestServer {
  private readonly server: Server
  private readonly url: Promise<string>
  constructor (...responses: Response[]) {
    if (responses == null) responses = [new Response(501, 'Not Implemented')]
    let count = 0
    const server = http.createServer((req, res) => {
      const r = responses[(count++) % responses.length]
      res.writeHead(r.code, { 'Content-Type': 'application/json' })
      res.write(JSON.stringify(r.payload))
      res.end()
    })
    const url: Promise<string> = new Promise((resolve, reject) => {
      server.listen(0, function () {
        const url = new URL('http://localhost')
        const info = server.address()
        if (info !== null && typeof info !== 'string' && info.port !== null) url.port = info.port.toString()
        resolve(url.toString())
      })
    })
    this.server = server
    this.url = url
  }

  public async ready (): Promise<string> {
    return await this.url
  }

  public close (): void {
    this.server.close()
  }
}