Clash Royale CLAN TAG#URR8PPP

How to create a list of unique items in JavaScript?

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 of string type.

My current way is to set keys of a object, then return the keys of that object
since the place the code can't use things like _.uniq for example.

_.uniq

I'd like to find a more elegant way to spell it than this.

function(keys, values, rereduce)
// values is a Array of Arrays
values = Array.concat.apply(null, values);
var uniq = ;
values.forEach(function(item) uniq[item] = true; );
return Object.keys(uniq);



group=true

6 Answers
6

Commonly, the approach you used is a good idea.
But I could propose a solution that will make the algorithm a lot faster.

function unique(arr)
var u = , a = ;
for(var i = 0, l = arr.length; i < l; ++i)
if(!u.hasOwnProperty(arr[i]))
a.push(arr[i]);
u[arr[i]] = 1;


return a;



As you can see we have only one loop here.

I've made an example that is testing both your and my solutions. Try to play with it.

for(var i=0; i<arr.length; ++i)

l

unique([, a:1]) === []

unique(['1', 1]) === ['1']

An alternative that's suitable for small lists would be to ape the Unix command line approach of sort | uniq:

sort | uniq

function unique(a)
return a.sort().filter(function(value, index, array)
return (index === 0) );



This function sorts the argument, and then filters the result to omit any items that are equal to their predecessor.

The keys-based approach is fine, and will have better performance characteristics for large numbers of items (O(n) for inserting n items into a hashtable, compared to O(n log n) for sorting the array). However, this is unlikely to be noticeable on small lists. Moreover, with this version you could modify it to use a different sorting or equality function if necessary; with hash keys you're stuck with JavaScripts notion of key equality.

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)
let u = ;
return arr.filter((v) =>
return u[v] = !u.hasOwnProperty(v);
);
;


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

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


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

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), );



This should work with anything, not just strings:

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

const set = new Set<any>();

for(let i = 0; i < a.length; i++)
if(!set.has(a[i]))
set.add(a[i]);



return Array.from(set.values());

;


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);






By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.