【前端芝士树】如何完成数组的扁平化 Array flattern？

let array = [1, [2, 3, 4]];
let arrayDeeper = [1, [2, [3, 4]]];

function flatten_1(arr) {
  return Array.prototype.concat.apply([], arr);
}

console.log(flatten_1(array));

function flattenDeeper_1(arr) {
  return arr.toString().split(',').map((item) => parseInt(item));
  //return arr.join(',').split(',').map((item) => parseInt(item));
}

function flattenDeeper_2(arr) {
  return arr.reduce((prev, current) => {
    return Array.prototype.concat(prev, Array.isArray(current) ? flattenDeeper(current) : current);
  })
}
function flattenDeeper_3(arr, result = []) {
  for (let item of arr) {
    if(Array.isArray(item)){
      flattenDeeper_2(item, result);
    }else{
      result.push(item);
    }
  }
  return result;
}
console.log(flattenDeeper_1(arrayDeeper));
console.log(flattenDeeper_2(arrayDeeper));
console.log(flattenDeeper_3(arrayDeeper));