js sort

有谁知道这是为什么吗？a-b就能升序，b-a就能降序????????

var numbers=[2,3,4,45,234,234,23,454,4,3,45,3,2,3]

numbers.sort(function(a,b){
　　return a-b
})

升序

return   [2, 2, 3, 3, 3, 3, 4, 4, 23, 45, 45, 234, 234, 454]

numbers.sort(function(a,b){
　　return b-a
})

降序

return [454, 234, 234, 45, 45, 23, 4, 4, 3, 3, 3, 3, 2, 2]