code: function first(arr, fn) {
    const l = arr.length;
    for (let i = 0; i < l; i += 1) {
      const res = fn(arr[i], i);
      if (res) {
        return res;
      }
    }
  
    return null;
  }
docs: Get the first truthy value returned by a mapper function
  @param {any[]} arr
  @param {function} fn