@tremorvideo/blink-cli/src/util/compare-browser-queries.js

const browserslist = require('browserslist')

const versionToNumber = (v) => {
  if (v === 'all') return 0
  if (v.includes('-')) v = v.slice(0, v.indexOf('-'))
  return +v
}

const formatBrowserNames = (q) => {
  return q.map((entry) => {
    return entry.replace(/(^[a-z_]+)(\s*[><=])/i, (a, b, c) => {
      b = browserslist(`last 1 ${b} version`)[0].split(' ')[0]
      return b + c
    })
  })
}

const getMinVersionObject = (obj, entry) => {
  if (/^[a-z_]+\s*>=/i.test(entry)) {
    let browser, version
    [browser, version] = entry.split(/\s*>=+\s*/)
    version = versionToNumber(version.trim())
    browser = browser.trim()
    if (!obj[browser] || obj[browser > version]) obj[browser] = version
  }
  return obj
}

module.exports = (queries) => {
  // convert any string queries to arrays
  queries = queries.map((q) => {
    if (Array.isArray(q)) return q
    q = q.split(/\s*,\s*/)
    return Array.isArray(q) ? q : [q]
  })

  // format browser names consistently
  queries = queries.map(formatBrowserNames)

  // get minimum version object for specified version in eah query
  // this is to protect against caniuse db not containing full list of versions
  const minSpecified = queries.map((q) => q.reduce(getMinVersionObject, {}))

  // generate a list of unique browsers for each query
  const supported = queries.map((q, index) => {
    return browserslist(q).reduce((list, browser) => {
      let version
      [browser, version] = browser.split(' ')
      version = versionToNumber(version)
      if (!list[browser] || list[browser] > version) list[browser] = version
      if (minSpecified[index][browser] && list[browser] > minSpecified[index][browser]) list[browser] = minSpecified[index][browser]
      return list
    }, {})
  })

  // count # occurences of each browser throughout each list for filtering
  const browserCount = supported.reduce((browsers, list) => {
    Object.keys(list).forEach((browser) => {
      if (!browsers[browser]) browsers[browser] = 0
      browsers[browser]++
    })
    return browsers
  }, {})

  // generate a consolidated minimum supported browser list
  const minSupported = supported.reduce((min, list) => {
    Object.keys(list).filter((browser) => browserCount[browser] === supported.length)
      .forEach((browser) => {
        if (!min[browser] || min[browser] < list[browser]) min[browser] = list[browser]
      })
    return min
  }, {})

  // convert the list of supported browsers back into a browserslist query
  const query = Object.keys(minSupported).map((browser) => {
    if (minSupported[browser] === 0) return `${browser} all`
    return `${browser} >= ${minSupported[browser]}`
  })

  return query
}