Pairwise suggested solution

Solution
function pairwise(arr, arg) {
  let sumIndices = 0;          // running sum of indices matched
  let matchedPairs = [];    // stores indices of matched pairs

  arr.forEach((elem, index) => {

     // check sum with array elements that follow elem
      for (let i = index+1; i < arr.length; i++) {

          if ((elem + arr[i]) == arg) {

              // check that array index hasn't been used already
              if (!matchedPairs.some(mp => (mp == index || mp == i))) {
                  sumIndices += (index + i);
                  matchedPairs.push(index, i)
              }  
         }  

      } 
  })

  return sumIndices;

}  // function pairwise

// test here
pairwise([1,4,2,3,0,5], 7);     // returns 11

Challenge: Pairwise

Link to the challenge: