const animals = ["cats", "dogs", "bunnies", "birds"];

const start_with_b = animals.filter(name => name.indexOf("b") === 0);

console.log(start_with_b); // ['bunnies', 'birds']

// function of filter (basic callback for filter)
const arr = [1, 3, 42, 2, 4, 5];
function filter (array, callback) {
  const callback_list = [];
  for (const i of array) {
    callback_list.push(callback(i));
  }
  return callback_list;
}
// modifyable callback function
function callback (num) {
  return Math.pow(num, 2);
}
console.log(filter(arr, callback));```