Алексей Алексеев

function _(array) {
  const result = [];
  array.forEach(element => 
  Array.isArray(element) 
  ? result.push(..._(element)) 
  : result.push(element));
  return result;
}

let array = [
    ["1", "2"],
    "3",
    ["4",  ["5", "6"] ]
]

console.log(_(array).join(' '))