Get all non-unique values (i.e.: duplicate/more than one occurrence) in an array

Question

I need to check a JavaScript array to see if there are any duplicate values. What\'s the easiest way to do this? I just need to find what the duplicated values are - I don\'

Answer 1

This is probably one of the fastest way to remove permanently the duplicates from an array 10x times faster than the most functions here.& 78x faster in safari

function toUnique(a,b,c){//array,placeholder,placeholder
 b=a.length;
 while(c=--b)while(c--)a[b]!==a[c]||a.splice(c,1)
}
var array=[1,2,3,4,5,6,7,8,9,0,1,2,1];
toUnique(array);
console.log(array);

1. Test: http://jsperf.com/wgu
2. Demo: http://jsfiddle.net/46S7g/
3. More: https://stackoverflow.com/a/25082874/2450730

if you can't read the code above ask, read a javascript book or here are some explainations about shorter code. https://stackoverflow.com/a/21353032/2450730

EDIT As stated in the comments this function does return an array with uniques, the question however asks to find the duplicates. in that case a simple modification to this function allows to push the duplicates into an array, then using the previous function toUnique removes the duplicates of the duplicates.

function theDuplicates(a,b,c,d){//array,placeholder,placeholder
 b=a.length,d=[];
 while(c=--b)while(c--)a[b]!==a[c]||d.push(a.splice(c,1))
}
var array=[1,2,3,4,5,6,7,8,9,0,1,2,1];

toUnique(theDuplicates(array));