react-query/examples/react/auto-refetching/src/pages/api/data.ts

import type { NextApiRequest, NextApiResponse } from 'next'

// an simple endpoint for getting current list
let list = ['Item 1', 'Item 2', 'Item 3']

export default async (
  req: NextApiRequest,
  res: NextApiResponse<typeof list>,
) => {
  if (req.query.add) {
    if (!list.includes(req.query.add)) {
      list.push(req.query.add)
    }
  } else if (req.query.clear) {
    list = []
  }

  await new Promise((r) => setTimeout(r, 100))

  res.json(list)
}