数组扁平化

function flatten(arr) {
  let result = [];

  for(let i = 0; i < arr.length; i++) {
    if(Array.isArray(arr[i])) {
      result = result.concat(flatten(arr[i]));
    } else {
      result.push(arr[i]);
    }
  }
  return result;
}

function flatten(arr) {
    return arr.reduce(function(prev, next){
        return prev.concat(Array.isArray(next) ? flatten(next) : next)
    }, [])
}

function flatten(arr) {
    while (arr.some(item => Array.isArray(item))) {
        arr = [].concat(...arr);
    }
    return arr;
}

arr.toString().split(',')

 arr.flat(Infinity);