How do Array.flat() and Array.flatMap() work?

const nested = [1, [2, [3, [4]]]];

nested.flat();         // [1, 2, [3, [4]]] — default depth 1
nested.flat(2);        // [1, 2, 3, [4]]
nested.flat(Infinity); // [1, 2, 3, 4] — fully flat

// flatMap = map + flat(1) — more efficient than separate calls
const sentences = ['Hello World', 'Foo Bar'];
sentences.flatMap(s => s.split(' ')); // ['Hello', 'World', 'Foo', 'Bar']

// vs two-step (less efficient)
sentences.map(s => s.split(' ')).flat(); // same result

// flatMap can filter + transform in one pass
const nums = [1, 2, 3, 4, 5];
nums.flatMap(n => n % 2 === 0 ? [n, n * 10] : []);
// [2, 20, 4, 40] — odds removed, evens doubled
// Return [] to skip, [val] to keep, [a, b] to expand one item into two