javascript - 我在最后一个函数中的数字旁边得到 NaN

Question

var john  = {
name: 'John Smith',
bills: [124,48,268,180,42],
tips : [],
final: [],
calcTips: function(){
    for(i = 0; i < this.bills.length;i++){
        if(this.bills[i] < 50){
            this.tips[i] = this.bills[i] * 0.2;
            this.final[i] = this.bills[i] + this.tips[i];
        } else if(this.bills[i] >= 50 && this.bills[i] <= 200){
            this.tips[i] = this.bills[i] * 0.15;
            this.final[i] = this.bills[i] + this.tips[i];
        } else{
            this.tips[i] = this.bills[i] * 0.1;
            this.final[i] = this.bills[i] + this.tips[i];
        }
    }
}

};

var mark  = {
name: 'Mark Smith',
bills: [77,375,110,45],
tips : [],
final: [],
calcTips: function(){
    for(i = 0; i < this.bills.length;i++){
        if(this.bills[i] < 100){
            this.tips[i] = this.bills[i] * 0.2;
            this.final[i] = this.bills[i] + this.tips[i];
        } else if(this.bills[i] >= 100 && this.bills[i] <= 300){
            this.tips[i] = this.bills[i] * 0.1;
            this.final[i] = this.bills[i] + this.tips[i];
        } else{
            this.tips[i] = this.bills[i] * 0.25;
            this.final[i] = this.bills[i] + this.tips[i];
        }
    }
}

};

function calcAverage(tips){
var sum = 0;
for(i = 0; i < tips.length ; i++){
    sum = tips[i] + sum;
}
return sum / tips.length;

}

mark.Average = calcAverage(mark.tips);
john.Average = calcAverage(john.tips);
mark.calcTips();
john.calcTips();
console.log(john);
console.log(mark);

除了平均值之外，函数 calcAverage 返回 NaN。我对其进行了编码，以便它将循环tips数组并将tips加上总和添加到sum变量中。有人可以解释一下这个问题吗？

Answer 1

据我所知，平均小费是在小费实际存在之前计算的。如果您移动上面提示的计算，它应该可以正常工作:

mark.calcTips();
john.calcTips();
mark.Average = calcAverage(mark.tips);
john.Average = calcAverage(john.tips);
console.log(john);
console.log(mark);