数组扁平化

数组扁平化函数flat：

var arr = ['a','b',['1','2',['3','4']],'c'];
        /*方法一：*/
        function flat(arr){
            var result = [];
            var each = function(arr){
                arr.forEach(function(i){
                    if(i instanceof Array){
                        each(i);
                    } else {
                        result.push(i);
                    }
                });
            }
            each(arr);
            return result.join(',');
        }
        var result = flat(arr);
        console.log(result);

         /*方法二：*/
        function flat(arr){
            var stringValue = Array.prototype.toString;
            Array.prototype.toString = function(){
                return this.join(',');
            }
            var result = arr + '';
            Array.prototype.toString = stringValue;
            return result;
        }

        /*方法三：*/
        function flat(){
            var Value = Array.prototype.valueOf;
            Array.prototype.valueOf = function(){
                return this.join(',');
            }
            var result = arr + '';
            Array.prototype.valueOf = Value;
            return result;
        }

        /*方法四：*/
        Array.prototype[Symbol.iterator] = function(){ //ES6中Symbol类型
            var arr = [].concat(this);
            var getfirst = function(array){
                var first = array.shift();
                return first;
            };
            return {
                next: function(){
                    var item = getfirst(arr);
                    if(item){
                        return {
                            value: item,
                            done: false
                        };
                    } else {
                        return {
                            done: true
                        };
                    }
                }
            }
        };
        function flat(arr){
            var result = [];
            for(var i in arr){
                result.push(i);
            }
            return result.join(',');
        }
        
        var result = flat(arr);
        console.log(result);