Basic JavaScript - Replace Loops using Recursion

Hello, I passed the test, but I have no idea what I’m doing.

Why does my code only sum the first n elements of an array arr? If it only works once, why can recursion alter the while and for loops?
What if I want to add the first two numbers of the array? What should I do?

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
}

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Challenge: Basic JavaScript - Replace Loops using Recursion

Link to the challenge:

I’m not sure I understand what you mean by “If it only works once”. Can you elaborate a little more?

The you would pass 2 as the second argument to the function. But I’m not sure if that is what you were asking. Again, please explain in a little more detail if it was not.

This task requires the function to sum the first n elements of an array arr . So if the array contains [2, 3, 4, 5], the function will return 2.
Did I misunderstand that it only works once because it only sums the first elements of the array and not the others? Can you explain to me exactly how this function works? Thank you in advance.

Remember, you call the function with two arguments: the array and how many numbers from the array you want to add together. So in your example above, if you called the function as sum([2, 3, 4, 5], 1) then it will return 2. But if you call it as sum([2, 3, 4, 5], 2) then you are telling it to add the first two numbers in the array and thus it will return 5.

I think you should explain it to me :slight_smile:

Let’s take the function call sum([2, 3, 4, 5], 2). What will happen when this function executes? Which return statement will it trigger? What will that return statement look like? Replace the variables names with their actual values.

Oh my God, I’ve got it now! Thank you so much. Initially, I just didn’t understand what sum(arr, n - 1) does. But now, with your guidance, I realize that sum(arr, n - 1) is the returned value of the function for the previous iteration.

That’s why the overall value will change depending on the second argument I assign to the function. Am I right?

I’m really sorry for my bad explanation throughout this entire discussion

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