filter objects from array that have the same value at given key

Question

Let's say we have an array that looks like this:

[
    {
        id: 0,
        name: 'A'
    },
    {
        id: 1,
        name:'A'
    },
    {
        id: 2,
        name: 'C'
    },
    {
        id: 3,
        name: 'B'
    },
    {
        id: 4,
        name: 'B'
    }
]

I want to keep only this objects that have the same value at 'name' key. So the output looks like this:

[
    {
        id: 0,
        name: 'A'
    },
    {
        id: 1,
        name:'A'
    },
    {
        id: 3,
        name: 'B'
    },
    {
        id: 4,
        name: 'B'
    }
]

I wanted to use lodash but I don't see any method for this case.

so, you want to keep object whose 'name' value exist in more than one object, right? — sanjiv saini
– sanjiv saini, Commented Sep 1, 2017 at 10:48
Could you clarify that? You only want to keep objects whose name exists at least twice in the array? Or that are grouped in pairs of two subsequent objects with the same name? — Sebastian Simon
– Sebastian Simon, Commented Sep 1, 2017 at 10:49
I only want to keep objects whose name exists at least twice in the array — sympi
– sympi, Commented Sep 1, 2017 at 10:50
@sympi Ah, okay, in this case, Rajesh’s answer should be the solution, though it doesn’t use lodash. — Sebastian Simon
– Sebastian Simon, Commented Sep 1, 2017 at 10:51
Is it important in which order the list every element comes? What happens if id:1 (name: 'A)' is id:5 instead? — Rickard Elimää
– Rickard Elimää, Commented Sep 1, 2017 at 10:58

Community · Accepted Answer · 2020-06-20 09:12:55Z

9

You can try something like this:

Idea:

Loop over the data and create a list of names with their count.
Loop over data again and filter out any object that has count < 2

var data = [{ id: 0, name: 'A' }, { id: 1, name: 'A' }, { id: 2, name: 'C' }, { id: 3, name: 'B' }, { id: 4, name: 'B' }];

var countList = data.reduce(function(p, c){
  p[c.name] = (p[c.name] || 0) + 1;
  return p;
}, {});

var result = data.filter(function(obj){
  return countList[obj.name] > 1;
});

console.log(result)

edited Jun 20, 2020 at 9:12

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answered Sep 1, 2017 at 10:49

Rajesh Dixit

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2 Comments

Rajesh Dixit Over a year ago

To be honest, I did check your answer(more than once), and as always, its something that still goes over my head. Will read it few more times. ;-) Also will try to do it in 1 loop. Thanks for pointing out.

Nina Scholz Over a year ago

it works with empty arrays and if it is later valid, then it gets an element with update. concat filters empty elements.

jonahe · Accepted Answer · 2019-05-02 21:08:16Z

5

A lodash approach that may (or may not) be easier to follow the steps of:

const originalArray = [{ id: 0, name: 'A' }, { id: 1, name: 'A' }, { id: 2, name: 'C' }, { id: 3, name: 'B' }, { id: 4, name: 'B' }];

const newArray =
      _(originalArray)
      	.groupBy('name') // when names are the same => same group.  this gets us an array of groups (arrays)
        .filter(group => group.length == 2) // keep only the groups with two items in them
        .flatten() // flatten array of arrays down to just one array
        .value();
        
console.log(newArray)

<script src="https://cdn.jsdelivr.net/npm/[email protected]/lodash.min.js"></script>

edited May 2, 2019 at 21:08

answered Sep 1, 2017 at 10:57

jonahe

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Comments

Faly · Accepted Answer · 2017-09-01 12:47:02Z

4

A shorter solution with array.filter and array.some:

var data = [ { ... }, ... ];  // Your array
var newData = data.filter((elt, eltIndex) => data.some((sameNameElt, sameNameEltIndex) => sameNameElt.name === elt.name && sameNameEltIndex !== eltIndex));
console.log("new table: ", newTable);

answered Sep 1, 2017 at 12:47

Faly

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2 Comments

jonahe Over a year ago

Nice solution, but the small size (long line) doesn't exactly help readability. The lambda with the some call could maybe be extracted to a named function?

Faly Over a year ago

Yes, named function can be used for these callbacks

Nina Scholz · Accepted Answer · 2017-09-01 11:21:25Z

2

You could use a hash table and a single loop for mapping the objects or just an empty array, then concat the result with an empty array.

var data = [{ id: 0, name: 'A' }, { id: 1, name: 'A' }, { id: 2, name: 'C' }, { id: 3, name: 'B' }, { id: 4, name: 'B' }],
    hash = Object.create(null),
    result = Array.prototype.concat.apply([], data.map(function (o, i) {
        if (hash[o.name]) {
            hash[o.name].update && hash[o.name].temp.push(hash[o.name].object);
            hash[o.name].update = false;
            return o;
        }
        hash[o.name] = { object: o, temp: [], update: true };
        return hash[o.name].temp;
    }));

console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

edited Sep 1, 2017 at 11:21

answered Sep 1, 2017 at 11:08

Nina Scholz

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