Find possible sum pairs in array

toSum.js

function toSum(numArray,sum){
  let pairs = [];
  let hash = [];
  for(let i = 0;i < numArray.length; i++){
    let counterPart = sum - numArray[i];
    if(hash.indexOf(counterPart) != -1){
      pairs.push([numArray[i],counterPart]);
    }
    hash.push(numArray[i]);
  }
  return pairs;
}

toSum([1,2,3,4,5,6],7);