1. 直接利用Array的flat()方法:

console.log([2, -5, 6, [6, -5, [2, 5], [8, 10, [6, 8], -3], 2], 5].flat(Infinity))

2. 一个简单好记得方法:

var arr = [12, 23, 34, [6, [6], [6,[7,[8]]]], [5,[5]], [4, [7]], [4, [[3]]], 45, 6];
arr.toString().split(",").map(function(item){
    return Number(item)
})
arr.toString().split(',').map(Number);

3. 利用递归纯手写

      function flatten(arr){
        var newArr = [];
        function fn(arr){
            for (let i = 0; i < arr.length; i++) {
                // console.log(arr[i],arr[i].length)
                if(arr[i].length){   //Array.isArray(arr[i])
                    fn(arr[i])
                }else{
                    newArr.push(arr[i])
                }
            }
        }
        fn(arr)
       // return newArr; //[2, -5, 6, 6, -5, 2, 5, 8, 10, 6, 8, -3, 2, 5]
        return newArr.sort(function(a,b){
            return a - b;
        }) ////[-5, -5, -3, 2, 2, 2, 5, 5, 6, 6, 6, 8, 8, 10]
    }
    console.log(flatten([2, -5, 6, [6, -5, [2, 5], [8, 10, [6, 8], -3], 2], 5]))
    

    function flatten(arr){
       if(arr.some(item=>Array.isArray(item))){
            let newArr = [];
            arr.forEach(function(item,index,arr){
                if(Array.isArray(item)){
                    newArr = newArr.concat(item)
                }else{
                    newArr.push(item)
                }
            })
            return flatten(newArr)
       }
       return arr.sort(function(a,b){
           return a - b;
       })
   }
 console.log(flatten([ [1, 2, 2], [3, 4, 5, 5], [6, 7, 8, 9, [11, 12, [12, 13, [14] ] ] ], 10]))
//[1, 2, 2, 3, 4, 5, 5, 6, 7, 8, 9, 10, 11, 12, 12, 13, 14]