Diff Two Arrays =>

I have been attempting to solve this problem using filter, after struggling I saw the answer is

    function diffArray(arr1, arr2) {
      return arr1
        .concat(arr2)
        .filter(
            item => !arr1.includes(item) || !arr2.includes(item)
        )
    }

how would I write this code without using =>? I have not learned it yet and would like to understand this fully first. Thank you!

Step by step is maybe easier to see:

item => !arr1.includes(item) || !arr2.includes(item)

same as

(item) => {
  return !arr1.includes(item) || !arr2.includes(item);
}

same as

function (item) {
 return !arr1.includes(item) || !arr2.includes(item);
}