数组求和的方法

• for in

function sum(arr){
	let count = 0;
	for(let i in arr){
	count += arr[i];
}
return count;
}

• for 循环

function sum(arr){
	let count = 0;
	for(let i = arr.length-1;i>=0;i--){
		count += arr[i];
}
return count;
}

• eval

function sum(arr){
	return eval(arr.join("+"));
}

• for Each 循环遍历

function sum(arr) {
    var count = 0;
    arr.forEach(function(item,index, arr) {
        s += item;
    }, 0);
  
    return count;
};