react-query/examples/react/optimistic-updates-ui/src/pages/api/data.ts

import type { NextApiRequest, NextApiResponse } from 'next'

type Todo = { id: string; text: string }

const items: Array<Todo> = []

export default async (req: NextApiRequest, res: NextApiResponse) => {
  await new Promise((r) => setTimeout(r, 1000))

  if (req.method === 'POST') {
    const text: string = req.body.text

    // sometimes it will fail, this will cause a regression on the UI

    if (Math.random() > 0.7) {
      res.status(500)
      res.json({ message: 'Could not add item!' })
      return
    }

    const newTodo = { id: Math.random().toString(), text: text.toUpperCase() }
    items.push(newTodo)
    res.json(newTodo)
    return
  } else {
    res.json({
      ts: Date.now(),
      items,
    })
  }
}