code: function findRoutes (dir, fileExtensions) {
    let files = fs.readdirSync(dir)
    let resolve = f => path.join(dir, f)
    let routes = files.filter(f => fileExtensions.indexOf(path.extname(f)) !== -1).map(resolve)
    let dirs = files.filter(f => fs.statSync(path.join(dir, f)).isDirectory()).map(resolve)
    return routes.concat(...dirs.map(subdir => findRoutes(subdir, fileExtensions)))
  }
docs: recursively searches for all js files inside a directory tree, and returns their full paths