common elements from 2 array

function intersection(firstArray, secondArray) {
 // array1 = [1,5,6,7,8];
 // array2 = [3,4,5,7,9];
  var hashmap = {};
  var intersectionArray = [];
  // assign 1 to all index in hashmap
  firstArray.forEach(function(element) {
    hashmap[element] = 1;
  });
  // Since we only want to push unique elements in our case... we can implement a counter to keep track of what we already added
  // now just check if second array has that index available and should equal to 1 
  secondArray.forEach(function(element) {
    if (hashmap[element] === 1) {
      intersectionArray.push(element);
      hashmap[element]++;
    }
  });
  return intersectionArray;
  // Time complexity O(n), Space complexity O(n)
}