Answer
flat(depth) flattens nested arrays up to the given depth (default 1). flatMap() maps each element first, then flattens the result by one level — equivalent to map().flat() but computed in a single, more efficient pass.
[[1,2],[3,4]].flat(); // [1,2,3,4]
[1,2,3].flatMap(n => [n, n*2]); // [1,2,2,4,3,6]Real-world example flatMap() to turn each order into multiple line-item records in one pass.