Removes duplicate values.
Alternatives: compare, map.
This is part of package extra-array.
array.unique(x, [fn]);
// x: an array
// fn: compare function (a, b)
const array = require('extra-array');
var x = [1, 2, 3, 4, 2, 3];
array.unique(x);
// [1, 2, 3, 4]
var x = [1, 2, 3, 4, -2, -3];
array.unique(x, (a, b) => Math.abs(a) - Math.abs(b));
// [1, 2, 3, 4]