anchous_nachos

let arr = [];
  function func(array) {
    for (let elem of array) {
      if (typeof elem == "object") {
        func(elem);
      } else {
        arr.push(elem);
      }
    }
    return arr;
  }
  console.log(func([1, [2, 7, 8], [3, 4, [5, [6, 7]]]]));

с использованием spread оператора:

function func(array) {
    let arr = [];

    if (typeof array == "object") {
      for (let elem of array) {
        arr.push(...func(elem));
      }
    } else {
      arr.push(array);
    }

    return arr;
  }

  console.log(func([1, [2, 7, 8], [3, 4, [5, [6, 7]]]]));

с методом array.concat()

function func(array) {
  let arr = [];

  if (typeof array == "object") {
    for (let elem of array) {
      arr = arr.concat(func(elem));
    }
  } else {
    arr.push(array);
  }

  return arr;
}

console.log(func([1, [2, 7, 8], [3, 4, [5, [6, 7]]]]));