Given an array of Directories/Filenames, find the longest filename

findLogestFileName1.js

function processDir(d, par = '') {
  let dirnm = par
  return d.map(item => {
    if (Array.isArray(item)) { // this is a folder
      return processDir(item, dirnm)
    }
    dirnm = par + '/' + item
    if (item.indexOf('.') > -1) { // this is a filename
      return dirnm
    }
    // return undefined
  })
  .filter(z => !!z) // remove dead-end folders
  .flat()
}

function findLongestStr(arr) {
  // return arr.slice().sort((a,b) => a.length - b.length).pop()
  const max = Math.max(...(arr.map(x => x.length)))
  const i = arr.findIndex(x => x.length === max)
  return arr[i]
}

let dirs = ["dir", ["sub1", ["file1.ext", "sub22222222222222222222222!"], "sub3", ["sub4", ["file2.ext"]]]]
// "/dir/sub3/sub4/file2.ext"
findLongestStr(processDir(dirs))