Function to return unique values from two arrays

uniqueIndices.js

function diff(arr1, arr2) {
  // put the arrays together
  // loop over the data
  combinedArr = arr1.concat(arr2);
  // filter over the combined array and compare the index and last index of each number
  // if they match then return that number
  let combedArray = combinedArr.filter((num) => {
    if (combinedArr.indexOf(num) === combinedArr.lastIndexOf(num)) {
      return num;
    }
  });
  return combedArray;
};


arr1 = [1,6,4];
arr2 = [3,1,2,4,9];

console.log(diff(arr1, arr2));