可以使用模块化%运算符对Javascript中的数组进行循环迭代。

与array.length和array.indexOf()一起，您可以使用执行您想要的操作

$scope.weekArray=["Sun","Mon","Tue","Wed","Thurs","Fri","Sat"];
var currentDay = "Mon";
var indexOfCurrentDay = $scope.weekArray.indexOf(currentDay);
var whatYouWant = (indexOfCurrentDay-previous_duration);
//note that modulo does not work with negative numbers.
while (whatYouWant < 0)
    whatYouWant += $scope.weekArray.length; 
whatYouWant = whatYouWant % $scope.weekArray.length;

这是一个正在工作的jsFiddle：https://jsfiddle.net/hf4zqnwu/3/

我可以想出两种方法。这看起来可疑地像一个家庭作业，所以我不打算为你拼写出来，如果不是，请道歉：P

模块算术

JavaScript有一个模块化运算符（%），它对类似的东西很有用。

15 % 7 = 1
21 % 7 = 0
5 % 7 = 5 

您可以使用它来"循环"到正确的值。请记住，回到7天和回到21天是一样的。

需要注意的重要一点是，如果你最终得到负数，模运算将不适用于你，所以为了避免问题，你可能想在你首先减去的数字上加7。

如果你做对了，你应该能够处理任何持续时间（即使返回的持续时间是12343434124123天）。如果做得不对，你可能会在任何超过7的时间内出现错误：）

使用计数器循环

使用for循环或while循环，倒计时需要返回的持续时间（如果您想返回5天，则从5到0）。从开始的一天开始，每次从你的指数中减去1。当索引超出数组中有效索引的范围时，循环返回到结束值。即，每次到达0时，返回到7。你最后得到的数字应该是你的目标指数。

我不会为你写代码，因为我觉得这是一项学校作业，但为什么不试试下面的x=今天之前的天数：

1. 获取当天的索引
2. 而x<7，x=x-7
3. 如果前几天===0，则返回当前日期
4. 如果当天的指数>x，则返回当天的天数-x
5. 否则，当日指数的返回日+（7-x）

只需使用模运算：

（indexOf_currentday-天）%7=indexOf_searchedDay

在你的例子中，星期一有索引1（1-5）%7=-4%7=4%7=4

//almost same but without array manipulation
var weeksize = 7;
var getDayBeforeIncluding = $scope.getDayBeforeIncluding = function(current, delta){ //current -number of day, delta- days       before 
    var realdelta = delta % weeksize;
    var result = current - realdelta;
    result = result > 0 ? result : (weeksize + result + 1); //+1 including
    return result;
 }
 var myday = getDayBeforeIncluding (1,5); // 5 days before monday including == 4
 console.log(""+myday+"== 4");

 //and more smart version both for strings and numbers with argument checking and both including and not including
 var weekArray = $scope.weekArray= "Sun","Mon","Tue","Wed","Thurs","Fri","Sat"];
 var weeksize = weekArray.length; //no more magic strings
 var getDayBefore = $scope.getDayBefore = function(current, delta, including){ //current -number of day, delta- days       before
    if(null==current){ 
       throw "no day provided";
    }   
    if(typeof(current)=='string'){
       current = weekArray.indexOf(current);
    }
    if(typeof(current)!='number'){
        throw "invalid day type";
    }
    if (current < 0 || current > weeksize) {
        throw "invalid current value";
    }
    var includedelta = including ? 1: 0;
    var realdelta = delta % weeksize;
    var result = current - realdelta;
    result = result > 0 ? result : (weeksize + result + includedelta ); //+1 including
    return result;
 }