gistfile1.js

  // Which files to traverse while finding id? Returns generator function.
  function traverser (id, dirs) {
    var head = [],
        inDir = [],
        dirs = dirs.slice(), // a copy of the original array
        exts = Object.keys(extensions);

    return function next () {
      var result = head.shift();
      if (result) { return result; }

      var gen = inDir.shift();
      if (gen) { head = gen(); return next(); }

      var dir = dirs.shift();
      if (dir !== undefined) {
        function direct (ext) { return path.join(dir, id + ext); }
        function index (ext) { return path.join(dir, id, 'index' + ext); }
        inDir = [
          function () { return exts.map(direct); },
          function () { return exts.map(index); }
        ];
        head = [path.join(dir, id)];
        return next();
      }
    };
  }