This post is Part 3 of a four part series:
Symmetric Difference of Multiple Arrays (Part 1)
Symmetric Difference of Multiple Arrays (Part 2)
Symmetric Difference of Multiple Arrays (Part 3) <– You are here
Symmetric Difference of Multiple Arrays (Part 4)

In this post, I focus on the third step of the solution: write a function that removes the duplicates from an array.

The function we want will take an array as an argument and filter out duplicates. Seems like a good time to use .filter(), eh?

There are other solutions for this which involves for loops, but in my opinion, a solution that makes use of a higher order function like .filter() is generally more concise, readable, and elegant.

Without further ado:

function removeDuplicates(arr) {
return arr.filter(function(item, pos) {
return arr.indexOf(item) == pos;

Our function will step through every element in the array and filter out any element that has an index which does not match it’s .indexOf(element) value. Since the .indexOf(element) method always returns the index of the first appearance of that element in the array, any element that appears more than once in the array will be removed.

That’s it for Part 3!

In Part 4, I’ll walk you through applying .reduce() to the arguments array using our symmetric difference helper function, then sorting and filtering the results (removing duplicates).