Why do I get an 'undefined' item after Array.reduce()?

I am trying to learn JavaScript. Now, I am doing experiments with arrays, JSON objects and the sort and reduce methods. The experiment is to find a nice solution to Codility.com’s NumberOfDiscIntersections task. I have seen the solution around, and I am trying to reproduce mine, so that I actually get the method.

My current code is almost done. What I ask here is: why am I getting a last ‘undefined’ element in the output of the execution of the following code?

This is the exact JavaScript code:

C = [2, 1, 1, 3, 2, 3];
answer = solution(C);
console.log(answer);

//circle radius varies from 0..2147483647
function solution(A) {
  //marker = {spot:0, openers:0, closers:0};
  const N = A.length;
  const points = [];
  let left = 0;
  let right = 0;

  // First, I count all borders
  for (i = 0; i < N; i++) {
    left = i - A[i];
    right = i + A[i];
    points.push({
      "position": left,
      "openers": 1,
      "closers": 0
    });
    points.push({
      "position": right,
      "openers": 0,
      "closers": 1
    });
  }
  console.log("points before sort:")
  for (item of points) {
    console.log(item);
  }
  points.sort((a, b) => {
    if (a.position < b.position) {
      return -1;
    } else if (a.position > b.position) {
      return 1;
    }
    return 0;
  });
  console.log("points after sort:")
  for (item of points) {
    console.log(item);
  }
  let parseable = points.reduce((acc, object) => {
    var index = acc.map((o) => o.position).indexOf(object.position);
    if (index == -1) {
      acc.push(object);
    } else {
      acc[index].openers += object.openers;
      acc[index].closers += object.closers;
    }
    return acc;
  }, []);
  console.log("points after reduce:")
  for (item of parseable) {
    console.log(item);
  }
}

Thia is my console output:

points before sort:
position: -2, openers: 1, closers: 0 }
position: 2, openers: 0, closers: 1 }
position: 0, openers: 1, closers: 0 }
position: 2, openers: 0, closers: 1 }
position: 1, openers: 1, closers: 0 }
position: 3, openers: 0, closers: 1 }
position: 0, openers: 1, closers: 0 }
position: 6, openers: 0, closers: 1 }
position: 2, openers: 1, closers: 0 }
position: 6, openers: 0, closers: 1 }
position: 2, openers: 1, closers: 0 }
position: 8, openers: 0, closers: 1 }
points after sort:
position: -2, openers: 1, closers: 0 }
position: 0, openers: 1, closers: 0 }
position: 0, openers: 1, closers: 0 }
position: 1, openers: 1, closers: 0 }
position: 2, openers: 0, closers: 1 }
position: 2, openers: 0, closers: 1 }
position: 2, openers: 1, closers: 0 }
position: 2, openers: 1, closers: 0 }
position: 3, openers: 0, closers: 1 }
position: 6, openers: 0, closers: 1 }
position: 6, openers: 0, closers: 1 }
position: 8, openers: 0, closers: 1 }
points after reduce:
position: -2, openers: 1, closers: 0 }
position: 0, openers: 2, closers: 0 }
position: 1, openers: 1, closers: 0 }
position: 2, openers: 2, closers: 2 }
position: 3, openers: 0, closers: 1 }
position: 6, openers: 0, closers: 2 }
position: 8, openers: 0, closers: 1 }
undefined

Why do I have this ‘undefined’ item in the end of the reduced array?

>Solution :

Because you are not returning anything from the solution function.

answer variable has default value undefined, which is the one getting logged in the console.

answer = solution(C); // `answer` is `undefined`
console.log(answer); // logs `undefined`

return something from the function, which answer variable can hold.

thanks