Pairwise bonfire FCC - very ugly solution

gistfile1.txt

function pairwise(arr, arg) {
  var indexRef = [];
  var pushIndex;
  // go through the arr
  for (var i = 0; i < arr.length; i ++) {
  
  // if that index is not already in indexRef, continue
    if (indexRef.indexOf(i) < 0) {
  // for that element, find its pair (arr.indexOf)
      pushIndex = arr.indexOf(arg-arr[i]);
      if (i !== arr.lastIndexOf(arg-arr[i])) {
      if ((pushIndex >= 0) && (indexRef.indexOf(pushIndex) <0)) {
        // if it exists, 
        // push those indexes to indexRef
        indexRef.push(pushIndex);
        indexRef.push(i);
        arr[i] = -1;
        arr[pushIndex] = -1;
      }
      }
    }
  // else, continue (nothing)
  }
  if (indexRef.length === 0) {
    return 0;
  }
  var sum = indexRef.reduce(function(a,b) {
    return a+b;
  });
  return sum;
}

pairwise([1,3,2,4], 4);