const arr = Array.from({ length: 400 }, () => ~~(Math.random() * 40));

let t1 = performance.now();
const res = arr.filter(el => el == 2);
let t2 = performance.now();

console.log('Filter', t2 - t1, res);

t1 = performance.now();
let res2 = (function(fn, a) {
  const f = []; //final
  for (let i = 0; i < a.length; i++) {
    if (fn(a[i])) {
      f.push(a[i]);
    }
  }
  return f;
})(el => el == 2, arr);
t2 = performance.now();

console.log('Fn', t2 - t1, res2);