arrays

const arr1 = [1, 2, 3, 4];
const result1 = arr1.flatMap(x => [x, x * 2]);
console.log(result1); // [1, 2, 2, 4, 3, 6, 4, 8]

const arr2 = ['a', 'b', 'c', 'd'];
console.log(arr2.at(-1)); // 'd'
console.log(arr2.at(-2)); // 'c'

const arr3 = [1, 2, 3, 4, 5];
const result2 = arr3.findLast(x => x % 2 === 0);
console.log(result2); // 4

const people = [
  { name: 'Alice', age: 21 },
  { name: 'Bob', age: 21 },
  { name: 'Charlie', age: 25 },
];

const groupedByAge = people.groupBy(person => person.age);
console.log(groupedByAge);
/*
{
  21: [
    { name: 'Alice', age: 21 },
    { name: 'Bob', age: 21 }
  ],
  25: [
    { name: 'Charlie', age: 25 }
  ]
}
*/

const arr4 = [3, 1, 4, 2];
const sorted = arr4.toSorted((a, b) => a - b);

console.log(arr4);    // [3, 1, 4, 2]
console.log(sorted); // [1, 2, 3, 4]