…Another solution with REDUCE… No code explanation needed … i hope …
function pairwise(arr, arg) { var indexSum = 0, len = arr.length, checkValue = NaN; if (len === 0) return indexSum; arr.reduce(function(acc,curr,index,array){ for (var i = index; i < len; i++){ //check for the same array's value if (checkValue === array[i]) { array[i] = NaN; }; if (acc + array[i] === arg) { indexSum += (index-1)+i; checkValue = acc; array[i] = NaN; break; }; }; acc = curr; return acc; }); return indexSum; } pairwise([1,4,2,3,0,5], 7);