remove all elements that occur more than once from array [duplicate]

Question

Before people think this is the same as all these other answers on Stack Overflow regarding removing duplicates can you please look at what is returned as opposed to what I am looking to do.

I want to remove all elements that occur more than once. I have tried the following:

const a = ['Ronaldo', 'Pele', 'Maradona', 'Messi', 'Pele'];

const uniqueArray = a.filter(function(item, pos) {
  return a.indexOf(item) == pos;
})

console.log(uniqueArray)

I want my uniqueArray to be

['Ronaldo', 'Maradona', 'Messi'];

Answer 1

In short: keep the value if the position of the first occurrence of the element (indexOf) is also the last position of the element in the array (lastIndexOf).

If the indexes are not equal then the value is duplicated and you can discard it.

const a = ['Ronaldo', 'Pele', 'Maradona', 'Messi', 
           'Pele', 'Messi', 'Jair', 'Baggio', 'Messi', 
           'Seedorf'];

const uniqueArray = a.filter(function(item) {
  return a.lastIndexOf(item) == a.indexOf(item);
});

console.log(uniqueArray);
/* output:  ["Ronaldo", "Maradona", "Jair", "Baggio", "Seedorf"] */

Codepen Demo