index.js

const MAX_HEADER_LENGTH = 2000
const THROW_ON_MAX_HEADER_LENGTH_EXCEEDED = false

function hasRel (x) {
  return x && x.rel
}

function intoRels (acc, x) {
  function splitRel (rel) {
    acc[rel] = Object.assign({}, x, { rel: rel })
  }

  x.rel.split(/\s+/).forEach(splitRel)

  return acc
}

function createObjects (acc, p) {
  // rel="next" => 1: rel 2: next
  const m = p.match(/\s*(.+)\s*=\s*"?([^"]+)"?/)
  if (m) acc[m[1]] = m[2]
  return acc
}

function parseLink (link) {
  try {
    const m = link.match(/<?([^>]*)>(.*)/)
    const linkUrl = m[1]
    const parts = m[2].split(';')
    const qry = {}
    // The origin is unused but it's required to parse relative URLs
    const url = new URL(linkUrl, 'https://example.com')

    for (const [key, value] of url.searchParams) {
      qry[key] = value
    }

    parts.shift()

    let info = parts.reduce(createObjects, {})
    info = Object.assign({}, qry, info)
    info.url = linkUrl
    return info
  } catch {
    return null
  }
}

function checkHeader (linkHeader, options) {
  if (!linkHeader) return false

  options = options || {}
  const maxHeaderLength = options.maxHeaderLength || MAX_HEADER_LENGTH
  const throwOnMaxHeaderLengthExceeded = options.throwOnMaxHeaderLengthExceeded || THROW_ON_MAX_HEADER_LENGTH_EXCEEDED

  if (linkHeader.length > maxHeaderLength) {
    if (throwOnMaxHeaderLengthExceeded) {
      throw new Error('Input string too long, it should be under ' + maxHeaderLength + ' characters.')
    } else {
      return false
    }
  }
  return true
}

function parseLinkHeader (linkHeader, options) {
  if (!checkHeader(linkHeader, options)) return null

  return linkHeader.split(/,\s*</)
    .map(parseLink)
    .filter(hasRel)
    .reduce(intoRels, {})
}

export { parseLinkHeader }