filter.js

function filter(args, func, cb) {
  const results = args.map(i => null);
  const count = 0;
  const error = null;
  args.map((item, j) => 
    func(item, (err, res) => {
      count++;
      if (err) {
        error = err;
      } else {
        results[j] = res;
      }
      
      if(count === args.length) {
        cb(error, results.filter(r => r));
      }
    });
  );
}