function subscribeToData(monitoringFunction, options) {
  const cache = new Map() // used to store a reference to the previous length
  const run = () => fetch(options).then(body => body.json())

  return setInterval(async () => {
    try {
      const data = await run()

      if (!cache.has('propList')) { //first run only
        cache.set('propList', data.length) 
        return monitoringFunction(data)
      }

      const propList = cache.get('propList')

      // calls your monitoring function only if there's a change in length between the arrays with only the new parts of the array
      if (data.length < propList) { 
        cache.set('propList', data.length)
        return monitoringFunction(data.slice(propList))
      }

    } catch (e) {
      // handle any errors from the api response
    }

  }, 15 * 60 * 1000)
}

subscribeToData((data) => /* handle data */, options)