--description--

Recursion is the concept that a function can be expressed in terms of itself. To help understand this, start by thinking about the following task: multiply the first n elements of an array to create the product of those elements. Using a for loop, you could do this:

  function multiply(arr, n) {
    let product = 1;
    for (let i = 0; i < n; i++) {
      product *= arr[i];
    }
    return product;
  }

However, notice that multiply(arr, n) == multiply(arr, n - 1) * arr[n - 1]. That means you can rewrite multiply in terms of itself and never need to use a loop.

  function multiply(arr, n) {
    if (n <= 0) {
      return 1;
    } else {
      return multiply(arr, n - 1) * arr[n - 1];
    }
  }

The recursive version of multiply breaks down like this. In the <dfn>base case</dfn>, where n <= 0, it returns 1. For larger values of n, it calls itself, but with n - 1. That function call is evaluated in the same way, calling multiply again until n <= 0. At this point, all the functions can return and the original multiply returns the answer.

Note: Recursive functions must have a base case when they return without calling the function again (in this example, when n <= 0), otherwise they can never finish executing.

--instructions--

Write a recursive function, sum(arr, n), that returns the sum of the first n elements of an array arr.

--hints--

sum([1], 0) should equal 0.

assert.equal(sum([1], 0), 0);

sum([2, 3, 4], 1) should equal 2.

assert.equal(sum([2, 3, 4], 1), 2);

sum([2, 3, 4, 5], 3) should equal 9.

assert.equal(sum([2, 3, 4, 5], 3), 9);

Your code should not rely on any kind of loops (for or while or higher order functions such as forEach, map, filter, or reduce.).

assert(
  !__helpers.removeJSComments(code).match(/for|while|forEach|map|filter|reduce/g)
);

You should use recursion to solve this problem.

assert(
  sum.toString().match(/sum\(.*\)/g).length > 1
);

--seed--

--seed-contents--

function sum(arr, n) {
  // Only change code below this line

  // Only change code above this line
}

--solutions--

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
}