Join pathes, return undefined if the resulting path is not within the path passed as the first argument.
The module extends node's path.posix and path.win32 with joinSafe methods, so it can be used like this:
const path = require('path')
/*const joinSafe = */require('path-join-safe') //joinSafe == path.joinSafe
let safe_path = path.posix.joinSafe('a/b/c', 'd') //returns 'a/b/c/d'
let unsafe_path = path.joinSafe('a/b/c', '..') //returns undefined