checksum2.js

function sumFinder(arr, sum){
  var differ = {}, 
      len = arr.length,
      substract;
  
  for(var i =0; i<len; i++){
     substract = sum - arr[i];

     if(differ[substract])
       return true;       
     else
       differ[arr[i]] = true;
  }
  return false;
}
sumFinder([6,4,3,2,1,7], 9);
// true
sumFinder([6,4,3,2,1,7], 2);
// false