How to pick latest item per user from an array in Javascript?

For example, I have an array like this:
[ 
{  
  "data": "some data",
  "user": "A",
  "createdAt":"2016-12-24",
},
{
  "data": "some data2",
  "user": "B",
  "createdAt":"2016-12-24",
},
{
  "data": "some data3",
  "user": "A",
  "createdAt":"2016-12-25",
},
{
  "data": "some data4",
  "user": "A",
  "createdAt":"2016-12-23",
},
{
  "data": "some data5",
  "user": "B",
  "createdAt":"2016-12-25",
},
{
  "data": "some data6",
  "user": "C",
  "createdAt":"2016-12-24",
},
]
I only need the latest created item per one user, so I need an array like this:
[ 
{
  "data": "some data3",
  "user": "A",
  "createdAt":"2016-12-25",
},
{
  "data": "some data5",
  "user": "B",
  "createdAt":"2016-12-25",
},
{
  "data": "some data6",
  "user": "C",
  "createdAt":"2016-12-24",
},
]
How can I do it with underscore or lodash?


Answer :

lodash provides the orderBy and uniqBy methods.
The key is to order by user then by createdAt in descending order. Since createdAt is a Date it needs to be treated as such. Conveiniently, the orderBy method allows one to specify either the property to order by or a function to evaluate (which returns the object to order by, in this case Date).
Below's what I came up with:
var data = [{ ... }],
    sorted = _.orderBy(
      data,
      ['user', function(d) {
        return new Date(d.createdAt)
      }],
      ['asc', 'desc'] // 'desc' orders by date descending
    ),
    uniqued = _.uniqBy(sorted, 'user');

source: http://stackoverflow.com/questions/41310265/how-to-pick-latest-item-per-user-from-an-array#41310442








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