Continuous check connection

available.js

const available = (function () {
  const url = `//${location.hostname}:${location.port}/ok`
  const available = {
    isAvailable: false,
    onChange: () => undefined
  }

  check()

  function check() {
    fetch(`${url}/?cache=${~~(Math.random() * 0x10000)}`, { method: 'get' })
      .then(({status}) => {
        let isAvailable = status >= 200 && (status < 300 || status === 304)
        update(isAvailable)
        setTimeout(check, (isAvailable ? 5 : 20) * 1000)
      })
      .catch(error => {
        update(false)
        setTimeout(check, 20 * 1000)
      })
  }

  function update(isAvailable) {
    if (available.isAvailable === isAvailable)
      return;
    available.onChange(isAvailable)
    available.isAvailable = isAvailable
  }

  return available
} ())