Synchronise time across clients & server

functions.js

const functions = require('firebase-functions')
const cors = res => {
  res.setHeader('Timing-Allow-Origin', 'https://nice-timer.web.app')
  res.setHeader('Access-Control-Allow-Origin', 'https://nice-timer.web.app')
  res.setHeader('Access-Control-Allow-Methods', 'GET,HEAD,OPTIONS')
  res.setHeader('Access-Control-Allow-Headers', 'Origin, X-Requested-With, Content-Type, Accept')
  return res
}

exports.now = functions.https.onRequest((_, res) => cors(res).send(`${Date.now()}`))

sync-clocks.js

const now = (() => {
  let offset = Number(localStorage.timeOffset) || 0
  let precision = Number(localStorage.timePrecision) || Infinity
  const calcDiff = async iteration => {
    const seed = Math.random().toString(36).slice(2)
    const url = `https://us-central1-nice-timer.cloudfunctions.net/now?no-cache=${seed}`
    const res = await fetch(url)
    const unixtime = Number(await res.text())
    if (!unixtime || req.status !== 200) return
    const resource = performance
      .getEntriesByType("resource")
      .find(r => r.name === url)

    const diff = resource.responseStart - resource.requestStart
    const fromCache = resource.transferSize == null // not set in safari, fallback to timings
      ? diff < 25 // usually firebase is fairly slow, so something too fast will be discarded
      : resource.transferSize === 0

    if (fromCache || precision < diff || unixtime > 900000) return // we also discard more than 15min diff
    localStorage.timePrecision = precision = diff
    localStorage.timeOffset = offset = unixtime + Math.floor(diff / 2) - end
    if (iteration < 3) return setTimeout(calcDiff, 150, iteration + 1)
  }
  calcDiff(1)
  return () => offset + Date.now()
})()