function parseArr(arr,res){
  var i=0;
  for(i=0;i<arr.length;i++){
    if(arr[i] instanceof Array){
      parseArr(arr[i],res);
    }else{
      res.push(arr[i]);
    }
  }
}
var a=[1,[2,[3,4]],5,6];
var res=[];
parseArr(a,res);

  var arr = ['mu','zi',['dig',['big','love']]]
  function flatten(arr){ 
    var res = []; 
    for(var i=0;i<arr.length;i++){
      if(Array.isArray(arr[i])){
        res = res.concat(flatten(arr[i]));
      }else{
        res.push(arr[i]);
      } 
    } 
    return res; 
  }
  console.log(flatten(arr))//["mu", "zi", "dig", "big", "love"]

//arr數(shù)組中的元素不能為字符串只能為數(shù)組
var newArr=arr.toString().split(',').map(function(ele){
  return +ele;
});
console.log(newArr)
alert(typeof arr[0]); //number

  var arr = [1, [2, [3, 4],[5,[6],[7,8]]]];
  var arrStr = arr.toString();
  console.log(arrStr);//1,2,3,4,5,6,7,8
  var strArr = arrStr.split(',');
  console.log(strArr)//["1", "2", "3", "4", "5", "6", "7", "8"]

Array.prototype.flatten=function(){
  return this.reduce(function(prev, cur) {
    var moreArr = [].concat(cur).some(Array.isArray); //判斷cur是不是一個(gè)數(shù)組
    return prev.concat(moreArr ? cur.flatten() : cur);
  },[]);
};
var arr=a.flatten();

  //合并二維數(shù)組
  var twoArr = [['mu','zi'],['dig','big'],['lucky','jiji']];
  var oneArr = twoArr.reduce(function(total,currentValue){
    // console.log(total)
    return total.concat(currentValue);
  })
  console.log(oneArr);//["mu", "zi", "dig", "big", "lucky", "jiji"]

  var arr = [1,2,3,4,5,6,7,8,9,10]
  var str = arr.reduce(function(prev,cur,index,arr){
    return prev + cur ;
  })
  console.log(str)//55

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

var arr = [1, [2, [3, 4]]];
console.log([].concat(...arr)); // [1, 2, [3, 4]]