function mockDispatch (opts, handler) {
  // Get mock dispatch from built key
  const key = buildKey(opts)
  const mockDispatch = getMockDispatch(this[kDispatches], key)

  mockDispatch.timesInvoked++

  // Here's where we resolve a callback if a callback is present for the dispatch data.
  if (mockDispatch.data.callback) {
    mockDispatch.data = { ...mockDispatch.data, ...mockDispatch.data.callback(opts) }
  }

  // Parse mockDispatch data
  const { data: { statusCode, data, headers, trailers, error }, delay, persist } = mockDispatch
  const { timesInvoked, times } = mockDispatch

  // If it's used up and not persistent, mark as consumed
  mockDispatch.consumed = !persist && timesInvoked >= times
  mockDispatch.pending = timesInvoked < times

  // If specified, trigger dispatch error
  if (error !== null) {
    deleteMockDispatch(this[kDispatches], key)
    handler.onError(error)
    return true
  }

  // Handle the request with a delay if necessary
  if (typeof delay === 'number' && delay > 0) {
    setTimeout(() => {
      handleReply(this[kDispatches])
    }, delay)
  } else {
    handleReply(this[kDispatches])
  }

  function handleReply (mockDispatches, _data = data) {
    // fetch's HeadersList is a 1D string array
    const optsHeaders = Array.isArray(opts.headers)
      ? buildHeadersFromArray(opts.headers)
      : opts.headers
    const body = typeof _data === 'function'
      ? _data({ ...opts, headers: optsHeaders })
      : _data

    // util.types.isPromise is likely needed for jest.
    if (isPromise(body)) {
      // If handleReply is asynchronous, throwing an error
      // in the callback will reject the promise, rather than
      // synchronously throw the error, which breaks some tests.
      // Rather, we wait for the callback to resolve if it is a
      // promise, and then re-run handleReply with the new body.
      body.then((newData) => handleReply(mockDispatches, newData))
      return
    }

    const responseData = getResponseData(body)
    const responseHeaders = generateKeyValues(headers)
    const responseTrailers = generateKeyValues(trailers)

    handler.onConnect?.(err => handler.onError(err), null)
    handler.onHeaders?.(statusCode, responseHeaders, resume, getStatusText(statusCode))
    handler.onData?.(Buffer.from(responseData))
    handler.onComplete?.(responseTrailers)
    deleteMockDispatch(mockDispatches, key)
  }

  function resume () {}

  return true
}
Mock dispatch function used to simulate undici dispatches