Javascript arrays can contain duplicates - which is fine most of the time, but can sometimes cause some issues. For example, if an array is supposed to only contain unique user IDs, but is taken from a data source which may contain duplicates:

let userIds = [ '123-123', '123-123', '234-234', '345-345' ]

In these cases, it can seem quite difficult to get only unique values. Is the best way to check every value against every other value in an array? That seems like a lot of work which should be simple. Fortunately, there is an easy way to make a unique array, and that is to use javascript sets.

A set can only contain unique values, so passing your array into a new Set() constructor will produce one set with unique values:

let userIds = [ '123-123', '123-123', '234-234', '345-345' ]
let uniqueUserIds = new Set(userIds)
console.log(uniqueUserIds) // Set(3) {'123-123', '234-234', '345-345'}

While sets have their own methods and these are described in my set guide here, sometimes arrays can be both more familiar and have more useful methods.

To convert your set back to an array, use the Array.from() method:

let userIds = [ '123-123', '123-123', '234-234', '345-345' ]
let uniqueUserIds = new Set(userIds)
let arrayUserIds = Array.from(uniqueUserIds)
console.log(arrayUserIds) // [ '123-123', '234-234', '345-345' ]

Now you have a perfectly unique set of array items - and you won't have to worry about processing times, or inaccuracy.