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Comparing arrays

Good day everyone,

I am trying to write a block of code that will compare two arrays and return a new array with any items not found in both of the original two arrays. This should work for numbers and words. For some reason I keep returning an empty array and I can't figure out why. Your help is much appreciated. Nick

function diff(arr1, arr2) {
  var newArr = [];

  var filteredOne = arr1.filter(firstFilter);
  var filteredTwo = arr2.filter(secondFilter);

  newArr = filteredOne.concat(filteredTwo);
  return newArr;

function firstFilter(value,arr2) {
  for (var i = 0; i<arr2.lenth; i++) {
    return value !== arr2[i];

function secondFilter(value,arr1){
  for (var i = 0; i<arr1.lenth; i++) {
    return value !== arr1[i];

diff([1, 2, 3, 5], [1, 2, 3, 4, 5]);

2 Answers

LaVaughn Haynes
LaVaughn Haynes
12,397 Points

From my understanding you can't use filter the way that you are using it. If you don't mind not using filter you can do it this way

<!DOCTYPE html>
<html lang="en">
    <meta charset="UTF-8">

        function getUniqueElements(a1, a2){

            var newArray = [];

            //loop through the first array
            for(var i=0; i < a1.length; i++){

                //if the element not present in 2nd array
                //push it to newArray
                if( a2.indexOf(a1[i]) < 0 ){


            //return newArray
            return newArray;


        function diff(arr1, arr2){

            var uniqueArray1 = getUniqueElements(arr1, arr2);
            var uniqueArray2 = getUniqueElements(arr2, arr1);
            return uniqueArray1.concat(uniqueArray2);


        var theDifference = diff([1, 2, 3, 5], [1, 2, 3, 4, 5]);


Bah I was trying to find a way to do it with filter, this is better

Thank you for your help. It is much appreciated. Nick.