const example1 = [[1, 1, 1, [1.4, 1.6]], 666, [2, 3, [[4], [5, 6]]]];

function flat(arr) {
  const stack = arr;
  const result = [];

  while (stack.length) {
    const current = stack.shift();

    if (Array.isArray(current)) {
      stack.unshift(...current);
    }
    else {
      result.push(current);
    }
  }

  return result;
}

console.assert(`${flat(example1)}`, '1,1,1,1.4,1.6,666,2,3,4,5,6');