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);