Javascript Recursion Confusion

[nextdev]

Tell us what’s happening:
Describe your issue in detail here.

I don’t get how it eventually equals to 9 when the code looks like it would equal to 2 over and over. can someone explain how that happens

   **Your code so far**

function sum(arr, n) {
 // Only change code below this line
if ( n <= 0) {
  return 0;
} else {
  return sum (arr, n - 1) + arr[n - 1];
}
 // Only change code above this line
}

   **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4896.75 Safari/537.36

Challenge: Replace Loops using Recursion

Link to the challenge:

freeCodeCamp.org

[kevinSmith]

First of all, there are a gazillion posts on recursion, please check them out. There are a lot of detailed breakdowns.

I’m not sure why you think “it would equal to 2 over and over”.

Whenever I am confused about code, I start logging things out:

function sum(arr, n) {
  console.log(n, 'entering sum')
  if (n <= 0) {
    console.log(n, 'base case reached, returning 0')
    return 0;
  } else {
    console.log(n, 'not base case, computing value to return')
    const val = sum(arr, n - 1) + arr[n - 1] 
    console.log(n, 'not base case, value to return', val)
    return val;
  }
}

console.log('final =', sum([2, 3, 4, 5], 3))

[nextdev]

Your explanation was not helpful. The reason I think it equals 2 over and over is because you get 2 for n then minus 1. 1+1 is 2.

[kevinSmith]

My explanation can only be as helpful as the question is explicit.

I still don’t understand:

you get 2 for n then minus 1

What is the context for this statement? I mean yes, when moving from the second function call (where n = 2) to the third function call, we do subtract 1 from n. I don’t see how that relates to your question.

1+1 is 2

Yes, that is a factually correct statement. I’m not sure how it applies here. Is one of those 1s supposed to be the 2-1 from your previous statement? I don’t see anywhere in this code where we add 1 and 1, either in number literals or in variable. If we break it down even further, we can see that:

function sum(arr, n) {
  console.log(n, 'entering sum')
  if (n <= 0) {
    console.log(n, 'base case reached, returning 0')
    return 0;
  } else {
    console.log(n, 'not base case, computing value to return')
    const x = sum(arr, n - 1)
    console.log(n, 'sum(arr, n - 1)', x)
    const y = arr[n - 1]
    console.log(n, 'arr[n - 1]', y)
    console.log(n, 'adding', x, 'and', y)
    const val = x + y 
    console.log(n, 'not base case, value to return', val)
    return val;
  }
}

console.log('final =', sum([2, 3, 4, 5], 3))

Have you run the logging code? Do that and trace through the values.

And just to repeat, recursion is a very confusing subject, especially for learners. It takes time. Search the forum - there are a lot of detailed explanations. Also checkout youtube - I think some aspects of recursion can be better understood visually.