Here's how you do it if you don't care about the order:
const array1 = [1, 2, 3]; const array2 = [2, 3, 4]; console.log([...new Set([...array1, ...array2])]); // prints [1, 2, 3, 4]
It merges two arrays first. Then it creates a set out of that merged array and lastly convers the set back out to an array.
I searched for a solution and all I found was dated or wrong. This oneliner works and I'm using it to make it possible to add a list of product versions to another list and I don't want to mutate existing arrays because of React state stuff.
If you want to see the ES5 version, check out this Babel repl.