Simple example of server side events with express in TypeScript

stream.ts

import express, { RequestHandler, Response } from 'express'
import bodyParser from 'body-parser'
import cors from 'cors'

interface Client {
  id: string
  res: Response
}

interface Item {
  id: string
  name: string
}

const app = express()

const itemMiddleware = (): RequestHandler => {
  const clientCount = 0
  const clients: Array<Client> = []
  const items: Array<Item> = []
  return (_, res, next): void => {
    res.locals.clientCount = clientCount
    res.locals.clients = clients
    res.locals.items = items

    next()
  }
}

app.use(cors())
app.use(bodyParser.json({}))
app.use(itemMiddleware())

app.get('/', (req, res) => {
  res.json({ message: 'it works' })
})

app.post('/item', (req, res) => {
  const {
    id,
    name
  } = req.body

  if (res.locals.items.find((item: Item) => item.id === id)) {
    res.status(400).json({
      message: `Item ${id} already exists`
    })

    return
  }

  const item: Item = {
    id,
    name
  }

  res.locals.items.push(item)

  res.locals.clients.forEach(
    (client: Client) => client.res.write(`data: ${JSON.stringify(item)}\n\n`)
  )

  res.json({
    message: 'OK',
    data: item
  })
})

app.get('/item', (req, res) => {
  res.json({
    message: 'OK',
    data: res.locals.items
  })
})

app.get('/item/feed', (req, res) => {
  res.setHeader('Cache-Control', 'no-cache')
  res.setHeader('Content-Type', 'text/event-stream')
  res.setHeader('Connection', 'keep-alive')
  res.flushHeaders()

  res.write('\n\n')

  const client: Client = {
    id: (++res.locals.clientCount).toString(),
    res
  }

  res.locals.clients.push(client)

  req.on('close', () => {
    res.locals.clients = res.locals.clients
      .filter((c: Client) => c.id !== client.id)
  })
})

app.get('/item/:id', (req, res) => {
  const {
    id
  } = req.params

  const item = res.locals.items.find((item: Item) => item.id === id)

  if (item) {
    res.json({
      message: 'OK',
      data: item
    })
  } else {
    res.status(404).json({ message: 'Not OK' })
  }
})

app.listen(3030, () => console.log('listening')) // eslint-disable-line