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

ES5 only (i.e., it needs a filter() polyfill for IE8 and below):

var arrayToFilter = [ 4, 5, 5, 5, 2, 1, 3, 1, 1, 2, 1, 3 ];

arrayToFilter.
    sort().
    filter( function(me,i,arr){
       return (i===0) || ( me !== arr[i-1] );
    });