shields/lib/regular-update.js

'use strict'

// Map from URL to { timestamp: last fetch time, data: data }.
let regularUpdateCache = Object.create(null)

// url: a string, scraper: a function that takes string data at that URL.
// interval: number in milliseconds.
// cb: a callback function that takes an error and data returned by the scraper.
function regularUpdate(
  {
    url,
    intervalMillis,
    json = true,
    scraper = buffer => buffer,
    options = {},
    request = require('request'),
  },
  cb
) {
  const timestamp = Date.now()
  const cached = regularUpdateCache[url]
  if (cached != null && timestamp - cached.timestamp < intervalMillis) {
    cb(null, cached.data)
    return
  }
  request(url, options, (err, res, buffer) => {
    if (err != null) {
      cb(err)
      return
    }

    let reqData
    if (json) {
      try {
        reqData = JSON.parse(buffer)
      } catch (e) {
        cb(e)
        return
      }
    } else {
      reqData = buffer
    }

    let data
    try {
      data = scraper(reqData)
    } catch (e) {
      cb(e)
      return
    }

    regularUpdateCache[url] = { timestamp, data }
    cb(null, data)
  })
}

function clearRegularUpdateCache() {
  regularUpdateCache = Object.create(null)
}

module.exports = {
  regularUpdate,
  clearRegularUpdateCache,
}