node_modules/tar/lib/strip-absolute-path.js

// unix absolute paths are also absolute on win32, so we use this for both
const { isAbsolute, parse } = require('path').win32

// returns [root, stripped]
// Note that windows will think that //x/y/z/a has a "root" of //x/y, and in
// those cases, we want to sanitize it to x/y/z/a, not z/a, so we strip /
// explicitly if it's the first character.
// drive-specific relative paths on Windows get their root stripped off even
// though they are not absolute, so `c:../foo` becomes ['c:', '../foo']
module.exports = path => {
  let r = ''

  let parsed = parse(path)
  while (isAbsolute(path) || parsed.root) {
    // windows will think that //x/y/z has a "root" of //x/y/
    // but strip the //?/C:/ off of //?/C:/path
    const root = path.charAt(0) === '/' && path.slice(0, 4) !== '//?/' ? '/'
      : parsed.root
    path = path.substr(root.length)
    r += root
    parsed = parse(path)
  }
  return [r, path]
}