Simplest solution so far

function uniteUnique(...arr) {
let full=[];
for(let keys of arr){
return [ Set(full)]; 

uniteUnique([1, 3, 2], [5, 2, 1, 4], [2, 1]);

Hi guru, welcome to the forum.
Could you provide some context? Which challenge is this code solving? Why, and compared to what, is it remarkably simple?

PS: I’ve edited your post for readability. When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make it easier to read.

Please use the “preformatted text” tool in the editor (</>) to add backticks around text.

See this post to find the backtick on your keyboard.
Note: Backticks are not single quotes.