Lodash: What's the opposite of `_uniq()`? -


the _.uniq() in lodash removes duplicates array:

var tst = [  { "topicid":1,"subtopicid":1,"topicname":"a","subtopicname1":"w" },  { "topicid":2,"subtopicid":2,"topicname":"b","subtopicname2":"x" },  { "topicid":3,"subtopicid":3,"topicname":"c","subtopicname3":"y" },  { "topicid":1,"subtopicid":4,"topicname":"c","subtopicname4":"z" }]  var t = _.uniq(tst, 'topicname')

this returns: 

[ {"topicid":1,"subtopicid":1,"topicname":"a","subtopicname1":"w" },    { topicid: 2, subtopicid: 2, topicname: 'b', subtopicname2: 'x' },   { topicid: 3, subtopicid: 3, topicname: 'c', subtopicname3: 'y' } ]

what's opposite of this? should return single object each duplicate object:

[ { topicid: 3, subtopicid: 3, topicname: 'c', subtopicname3: 'y' } ]

i don't think there's built in method, here's should job:

function dupesonly(arr, field) {     var seen = {},         ret = [];      arr.foreach(function(item) {         var key = item[field],             val = seen[key];          if (!val) {             seen[key] = val = {                 initial: item,                 count: 0             }         }          if (val.count === 1) {             ret.push(val.initial);         }         ++val.count;     });      return ret; }

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