index.js
function normalizeString(path, allowAboveRoot) {
  var res = ''
  var lastSegmentLength = 0
  var lastSlash = -1
  var dots = 0
  var code
  for (var i = 0; i <= path.length; i += 1) {
    if (i < path.length) code = path.charCodeAt(i)
    else if (code === CHAR_FORWARD_SLASH) break
    else code = CHAR_FORWARD_SLASH
    if (code === CHAR_FORWARD_SLASH) {
      if (lastSlash === i - 1 || dots === 1) {
        // NOOP
      } else if (lastSlash !== i - 1 && dots === 2) {
        if (
          res.length < 2 ||
          lastSegmentLength !== 2 ||
          res.charCodeAt(res.length - 1) !== CHAR_DOT ||
          res.charCodeAt(res.length - 2) !== CHAR_DOT
        ) {
          if (res.length > 2) {
            var lastSlashIndex = res.lastIndexOf('/')
            if (lastSlashIndex !== res.length - 1) {
              if (lastSlashIndex === -1) {
                res = ''
                lastSegmentLength = 0
              } else {
                res = res.slice(0, lastSlashIndex)
                lastSegmentLength = res.length - 1 - res.lastIndexOf('/')
              }
              lastSlash = i
              dots = 0
              continue
            }
          } else if (res.length === 2 || res.length === 1) {
            res = ''
            lastSegmentLength = 0
            lastSlash = i
            dots = 0
            continue
          }
        }
        if (allowAboveRoot) {
          if (res.length > 0) res += '/..'
          else res = '..'
          lastSegmentLength = 2
        }
      } else {
        if (res.length > 0) res += '/' + path.slice(lastSlash + 1, i)
        else res = path.slice(lastSlash + 1, i)
        lastSegmentLength = i - lastSlash - 1
      }
      lastSlash = i
      dots = 0
    } else if (code === CHAR_DOT && dots !== -1) {
      ++dots
    } else {
      dots = -1
    }
  }
  return res
}
function normalizeString(path, allowAboveRoot) {
  var res = ''
  var lastSegmentLength = 0
  var lastSlash = -1
  var dots = 0
  var code
  for (var i = 0; i <= path.length; i += 1) {
    if (i < path.length) code = path.charCodeAt(i)
    else if (code === CHAR_FORWARD_SLASH) break
    else code = CHAR_FORWARD_SLASH
    if (code === CHAR_FORWARD_SLASH) {
      if (lastSlash === i - 1 || dots === 1) {
        // NOOP
      } else if (lastSlash !== i - 1 && dots === 2) {
        if (
          res.length < 2 ||
          lastSegmentLength !== 2 ||
          res.charCodeAt(res.length - 1) !== CHAR_DOT ||
          res.charCodeAt(res.length - 2) !== CHAR_DOT
        ) {
          if (res.length > 2) {
            var lastSlashIndex = res.lastIndexOf('/')
            if (lastSlashIndex !== res.length - 1) {
              if (lastSlashIndex === -1) {
                res = ''
                lastSegmentLength = 0
              } else {
                res = res.slice(0, lastSlashIndex)
                lastSegmentLength = res.length - 1 - res.lastIndexOf('/')
              }
              lastSlash = i
              dots = 0
              continue
            }
          } else if (res.length === 2 || res.length === 1) {
            res = ''
            lastSegmentLength = 0
            lastSlash = i
            dots = 0
            continue
          }
        }
        if (allowAboveRoot) {
          if (res.length > 0) res += '/..'
          else res = '..'
          lastSegmentLength = 2
        }
      } else {
        if (res.length > 0) res += '/' + path.slice(lastSlash + 1, i)
        else res = path.slice(lastSlash + 1, i)
        lastSegmentLength = i - lastSlash - 1
      }
      lastSlash = i
      dots = 0
    } else if (code === CHAR_DOT && dots !== -1) {
      ++dots
    } else {
      dots = -1
    }
  }
  return res
}
Resolves . and .. elements in a path with directory names