var arr =['a', ['b', 'c'], 2, ['d', 'e', 'f'], 'g', 3, 4];
function flat(arr) {
 return arr.toString().split(',').map(function(item){
    return Number(item)
 })
}
console.log(flat(arr))

var arr = ['a', ['b', 'c'], 2, ['d', 'e', 'f'], 'g', 3, 4];
// 方法二：toString（格式转换）
var flag = function(arr) {
	let toString = Array.prototype.toString;
	Array.prototype.toString = function() {
		return this.join(',');
	};
	let result = arr + '';
	Array.prototype.toString = toString;
	return result;
};

console.log(flag(arr));

// 方法三：valueOf(格式转换)
Array.prototype.valueOf = function() {
	return this.join(',');
};

var flat = function(arr) {
	return arr + '';
};
console.log(flat(['a', ['b', 'c'], 2, ['d', 'e', 'f'], 'g', 3, 4]));

function flat(arr) {
	return newArr = arr.reduce((a, b) => {
		return a.concat(b)
	}, [])
}
var arr = ['a', ['b', 'c'], '2', ['d', 'e', 'f'], 'g', 3, 4];
console.log(flat(arr));

function flat(array) {
	var result = [];
	var each = function(arr) {
		arr.forEach(item => {
			if (item instanceof Array) {
				each(item);
			} else {
				result.push(item);
			}
		});
	};
	each(array);
	return result.join(',');
}
var arr = ['a', ['b', 'c', [7, 8]], 2, ['d', 'e', 'f'], 'g', 3, 4];
console.log(flat(arr));

// Iterator
Array.prototype[Symbol.iterator] = function() {
	let arr = [].concat(this);
	// arr=['a', ['b', 'c'], '2', ['d', 'e', 'f'], 'g', 3, 4]
	let getFirst = function(array) {
		let first = array.shift();
		if (first instanceof Array) {
			if (first.length > 1) {
				arr = first.slice(1).concat(array);
			}
			first = first[0];
		}
		return first;
	};
	return {
		next: function() { //类似与遍历
			let item = getFirst(arr);
			if (item) {
				return {
					value: item,
					done: false,
				};
			} else {
				return {
					done: true,
				};
			}
		},
	};
};
var flat = function(arr) {
	let r = [];
	for (let i of arr) {
		r.push(i);
	} // i 已经是单个元素
	return r.join(',');
};
var arr = ['a', ['b', 'c'], '2', ['d', 'e', 'f'], 'g', 3, 4];
console.log(flat(arr));