Sequential Execution: Crawling of Links

function spiderLinks (currentUrl, body, nesting, cb) {
  if (nesting === 0) {
    return process.nextTick(cb)
  }
  const links = getPageLinks(currentUrl, body)                //(1)
  if (links.length === 0) {
    return process.nextTick(cb)
  }
  function iterate (index) {                                  //(2)
    if (index === links.length) {                             
          return cb() 
    }
    spider(links[index], nesting - 1, function (err) {        //(3)
      if (err) {
        return cb(err)
      }
      iterate(index + 1)
    })
  }
    iterate(0)                                                //(4) 
}