common-dir.js

function commonPrefixLen(names, sep) {
  const sc = sep.charCodeAt(0);
  for (let pos = 0; ; pos++) {
    for (let i = 0; i < names.length; i++) {
      let c = names[i].charCodeAt(pos)
      if (c && c == names[0].charCodeAt(pos)) {
        continue;
      }
      while (pos > 0 && names[0].charCodeAt(--pos) != sc) {}
      return pos;
    }
  }
  return 0;
}

let inpaths = [
  '/foo/bar/cat/bar/bobso',
  '/foo/bar/cat/bar/b',
  '/foo/bar/cat/ken/lolcat',
  '/foo/bar/cat/a',
  '/foo/bar/cat',
]
console.log(inpaths[0].substr(0, commonPrefixLen(inpaths, '/')))
// output: "/foo/bar"

inpaths = [
  '/foo/bar/cat/bar/bobso',
  '/foo/bar/cat/bar/b',
  '/foo/bar/cat/ken/lolcat',
  '/foo/bar/cat/a',
  '/bar',
]
console.log(inpaths[0].substr(0, commonPrefixLen(inpaths, '/')))
// output: ""