How to remove duplicates in JS.

remove_duplicates.js

// Reference: https://codepen.io/macmladen/pen/XBwgEa/

let arr = [1, 2, 3, 4, 5, 1, 2 ,3, 4, 5];
// Set, this is a simple and plain method.
[...new Set(arr)];

// filter, a built-in method based on the filter function.
arr.filter((item, index) => arr.indexOf(item) === index);
// Retrieve the duplicate values
arr.filter((item, index) => arr.indexOf(item) !== index);

// reduce. a built-in method based on the reduce function.
arr.reduce((unique, item) => unique.includes(item) ? unique : [...unique, item], []);