JS: Map.groupBy

(new in JS: ECMAScript 2024)

Map.groupBy(iterable, f)

• Group items in iterable by a function's return value.
• Return a map.
• Keys are return value of f.
• Each key's value is an array of items in iterable.

// group by mod 3

const xx = [1, 2, 3, 4, 5, 6, 7];
const yy = Map.groupBy(xx, (x) => x % 3);
console.log(yy);
// Map(3) { 1 => [ 1, 4, 7 ], 2 => [ 2, 5 ], 0 => [ 3, 6 ] }