Solution2.js

function sumOfTwoNum(array,target){
  let storeMap = {};

  for(let arr of array){
    
    if(storeMap[arr]){
      return true;
    }
    storeMap[target - arr] = arr;
  }
  return false;
}

console.log(sumOfTwoNum([10,15,3,7],18));

// 1st iteration storeMap = {8:10}             - check whether key 10 is exist in the map already (if(storeMap[arr]))
// 2nd iteration storeMap = {8:10,3:15}        - check whether key 15 is exist in the map already (if(storeMap[arr]))
// 3rd iteration storeMap = {8:10,3:15}        - check whether key 3 is exist in the map already (if(storeMap[arr]))
// yes 3 is exist in the map,so 3 + 15(current element) = 18 return true.