How can I remove all duplicated elements in an array (including first occurrence)

Question

I was studying JS in CodeWars and I didn't find a method to remove all duplicated elements in an array. I need to do exactly this:

a = [1,2,2,2,3,4,5,6,6,7] b = [1,2,7,8,9]

Return a unique array = [3,4,5,8,9] delete all the duplicated items, including the first occurrence

How can I do this? I already use for, if, forEach, but no success.

Answer 1

You may simply

• count the occurences of each element (to preserve the original element types, you may apply Array.prototype.reduce() together with Map against merged array)
• then, filter out those that are seen more than once:

const  a = [1,2,2,2,3,4,5,6,6,7],
       b = [1,2,7,8,9],
       
       uniques = [
         ...[...a, ...b]
          .reduce((acc,item) => 
            (acc.set(item, (acc.get(item)||0)+1), acc), new Map)
          .entries()
       ].reduce((acc, [key, value]) => 
        (value === 1 && acc.push(key), acc), [])
      
    
console.log(uniques)

.as-console-wrapper {min-height:100%}