Given a list of n integers arr[0..(n-1)], determine the number of different pairs of elements within it which sum to k

I’m tacking this problem and I can’t seem to arrive at the correct solution. The question is:

“Given a list of n integers arr[0..(n-1)], determine the number of different pairs of elements within it which sum to k. If an integer appears in the list multiple times, each copy is considered to be different; that is, two pairs are considered different if one pair includes at least one array index which the other doesn’t, even if they include the same values.”

My approach is that I’m building a map that contains each number in the array and the number of times it occurs. Then I iterate over the map to find my answer.

function numberOfWays(arr, k) {
  let output = 0;
  let map = {};

  // put values and # of occurences into map
  for(let i = 0; i < arr.length; i++) {
    let key = arr[i];
    if(!(key in map)) {
      map[key] = 1;
    } else {
      map[key]++;
    }
  }
  
  for(let key in map) {
    let difference = k-key
    if((difference) in map) {
      if(k/2 === key) {
        output += map[key]*(map[key]-1)/2;
      } else {
        output += map[key] * map[key] / 2;  // divide by 2 so that pairs aren't counted twice
      }
    }
  }

  return output;
}

The two test cases are:
var arr_1 = [1, 2, 3, 4, 3]; expected result: [2] — I’m getting [3]

var arr_2 = [1, 5, 3, 3, 3]; expected result: [4] — I’m getting [5.5]

I’m definitely doing something wrong in my calculations, but I can’t seem to wrap my ahead around it.