How to create a list of unique items in JavaScript?

Question

In my CouchDB reduce function I need to reduce a list of items to the unique ones.

Note: In that case it\'s ok to have a list, it will be a small number of items

Answer 1

I find the other answers to be rather complicated for no gain that I can see.

We can use the indexOf method of the Array to verify if an item exists in it before pushing:

const duplicated_values = ['one', 'one', 'one', 'one', 'two', 'three', 'three', 'four'];
const unique_list = [];

duplicated_values.forEach(value => {
  if (unique_list.indexOf(value) === -1) {
    unique_list.push(value);
  }
});

console.log(unique_list);

That will work with any type of variable as well, even objects (given the identifier actually reference the same entity, merely equivalent objects are not seen as the same).

Answer 2

This is an old question, I know. However, it is at the top of some google searches, so I wanted to add that you can combine the answers from @RobHague and @EugeneNaydenov using the following:

function unique(arr) {
  const u = {};
  return arr.filter((v) => {
    return u[v] = !u.hasOwnProperty(v);
  });
};

You can also ignore undefined values (often handy) by adding:

function unique(arr) {
  const u = {};
  return arr.filter((v) => {
    return u[v] = (v !== undefined && !u.hasOwnProperty(v));
  });
};

You can play with this solution here: https://jsfiddle.net/s8d14v5n/

Answer 3

This should work with anything, not just strings:

export const getUniqueList =  (a: Array<any>) : Array<any> => {

  const set = new Set<any>();

  for(let v of a){
      set.add(v);
  }

  return Array.from(set);

};

the above can just be reduced to:

export const getUniqueValues = (a: Array<any>) => {
   return Array.from(new Set(a));
};

:)

Answer 4

Using Object.keys will give you strings if you put in integer arguments (uniq([1,2,3]) => ['1','2','3']. Here's one with Array.reduce:

function uniq(list) {
    return list.reduce((acc, d) => acc.includes(d) ? acc : acc.concat(d), []);
}

Answer 5

The best method seem to be using ES6 and Set. Single line and faster* than above according to fiddle

    
const myList = [1,4,5,1,2,4,5,6,7];
const unique = [...new Set(myList)];
    
console.log(unique);

*tested in safari

Answer 6

what about

    function unique(list) {
      for (i = 0; i<list.length; i++) {
        for (j=i+1; j<list.length; j++) {
          if (list[i] == list[j]) {
            list.splice(j, 1);
          }
        }
      }
    }