flatten の実装

flatten.js

const target = [1, [2], [3, [4]], [], [[5]]];

function flatten(arr) {
  const result = [];
  for (let i = 0; i < arr.length; i++) {
    const value = arr[i];
    if (typeof value === 'number') {
      result.push(value);
      continue;
    }
    for(let x = 0; x < value.length; x++) {
      result.push(value[x]);
    }
  }
  const hasArray = result.some(item => Array.isArray(item));
  if (hasArray) {
    return flatten(result);
  };
  return result;
}

console.log(flatten(target));