lodash - Count duplicate and then remove them

Question

I am trying to use lodash to first count how many duplicates are in an array of objects and the remove the duplicate (keeping the counter).

The code I have so far se

Answer 1

You're looking for a reduce function, which is widely available in native JS. Here's a full code sample that does the job without Lodash:

    const inputArray = [
        {id: 1, name: "test"},
        {id: 2, name: "test 2"},
        {id: 3, name: "test 3"},
        {id: 4, name: "test 4"},
        {id: 1, name: "test "},
        {id: 2, name: "test 2"}
    ];

    const uniqueArrayWithCounts = inputArray.reduce((accum, val) => {
        const dupeIndex = accum.findIndex(arrayItem => arrayItem.id === val.id);

        if (dupeIndex === -1) {
          // Not found, so initialize.
          accum.push({
            qty: 1,
            ...val
          });
        } else {
          // Found, so increment counter.
          accum[dupeIndex].qty++;
        }
        return accum;
    }, []);

    console.log(uniqueArrayWithCounts);

Here's a good way to think about this:

1) Do you want the output array to be the same size (e.g. 1:1, every input has an output)? Then you're going to want to use map.

2) Do you want the output array to be a different size (usually smaller)? Then you're going to want to use reduce (or filter, etc).

Because we want to remove duplicates, that should lead you to use #2. That'll at least get you started going down the right path next time!

Answer 2

I would use groupBy() to group all items with the same ID into separate arrays, then only keep one from each individual array, setting qty to its length.

const array = [
  {id: 1, name: "test"},
  {id: 2, name: "test 2"},
  {id: 3, name: "test 3"},
  {id: 4, name: "test 4"},
  {id: 1, name: "test "},
  {id: 2, name: "test 2"}
];

const result = _(array).groupBy('id').values().map(
  (group) => ({...group[0], qty: group.length})
);

console.log(result);

<script src="https://unpkg.com/lodash@4.17.4/lodash.js"></script>

Edit Updated to make it shorter, and prevent mutating the original array elements.

Answer 3

A more functional approach could be this one, that is basically the same logic than the accepted answer but using lodash/fp

const array = [
  { id: 1, name: 'test' },
  { id: 2, name: 'test 2' },
  { id: 3, name: 'test 3' },
  { id: 4, name: 'test 4' },
  { id: 1, name: 'test' },
  { id: 2, name: 'test 2' },
]

_.flow(
  _.groupBy()('id'),
  _.map(keys => ({
    ...(_.head(keys)),
    qty: keys.length,
  })),
  _.tap(console.log)
)(array)