在mongodb mapreduce的reduce函数中，参数values是一个数组，可以用values.forEach进行遍历。
function(key, values){
//values 是一个数组
values.forEach(function(val){
//val即为数组里的元素
});
}
但是values比较大时，values会被会被拆分存储，在原本Reduce函数中的forEach只遍历了第一层的数据，无法遍历所有的元素。那请问如何遍历values里的所有元素？
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There is also a parameter scope in mapreduce, which can place variables used in js functions. You can temporarily store value data in it, so that you can obtain data across a single reduce function. However, since the values are split, it means that the amount of data in the values is large. If you just forcibly combine all the values, it is not a good idea, and the memory will be tight. It is better to optimize the map function and reduce the amount of values data.