js 这个递归函数是怎么走的

Question

function sum(arr, n) {
    if(n <= 0) {
      return 0
    }
    else {
      return sum(arr, n - 1) + arr[n - 1]
    }
}
sum([2, 3, 4, 5], 3)

Answer 1

sum([2, 3, 4, 5], 3)，n=3 ，执行else 也就是 sum([2, 3, 4, 5], 3) = sum([2, 3, 4, 5], 2) + 4
sum([2, 3, 4, 5], 2)，n=2，这行else 也就是 sum([2, 3, 4, 5], 2) = sum([2, 3, 4, 5], 1) + 3
sum([2, 3, 4, 5], 1)，n=1，执行else 也就是 sum([2, 3, 4, 5], 1) = sum([2, 3, 4, 5], 0) + 2
sum([2, 3, 4, 5], 0) ，n=0，执行if 返回0
然后逐个往回带