const arr = [1, [2, 3], [4, [5, 6]], 7];
const a = arr.flat();
const b = arr.flat(2);
const c = arr.flatMap(x => Array.isArray(x) ? x : [x, x]);
const d = [1, 2, 3].flatMap(x => [[x]]);
console.log(JSON.stringify([a, b, c, d]));