Making an Array unique in JavaScript

So, you have an Array that you need to make unique in Javascript? But how?

Given the following items:

items = [
  'item1',
  'item2',
  'item3',
  'item3',
  'item4',
  'item4',
  'item4'
]

Using ES6, we can use the unique nature of Set.

const newItems = [...new Set(items)]
// [ 'item1', 'item2', 'item3', 'item4' ]

Set creates a unique mapping, which we convert to a new Array using the spread operator '...' to give us the new list. Neat!

If you're stuck supporting ES5/older browsers (IE9+), you can use the less concise filter trick.

var newItems = items.filter(function (item, idx, arr) {
  return arr.indexOf(item) == idx
})
// [ 'item1', 'item2', 'item3', 'item4' ]

How does this work? indexOf returns the index of the first matching item, so only the first occurrence will ever match the index in the loop.