Underscore.js find unique values in array of objects; Return unique items and their count

Question

I am using _underscore.js to find all unique items in an array, but I can't figure out how to also get the number of unique items returned.

_PERSONARRAY = [{name:"tom",age:7}{name:"john",age:9}{name:"becky",age:2}{name:"sam",age:7}]

_UNIQUEAGEARRAY = _.chain(_PERSONARRAY).map(function(person) { return person.age }).uniq().value();

In this case _UNIQUEAGEARRAY will equal:

[7,9,2]

What I actually need returned is something like:

[{uniqueAge:7,numberOfPeople:2}{uniqueAge:9,numberOfPeople:1}{uniqueAge:2,numberOfPeople:1}]

Thanks for help. Also, I'm assuming _underscore.js is quick at doing this?? If it's stupid slow tell me cause I'd be open to other solutions.

GammaGames · Accepted Answer · 2019-04-06 04:12:43Z

82

A nice solution is to use the optional iterator function to underscore's uniq function:

let people = [
  {name: "Alice", age: 21}, 
  {name: "Bob", age: 34},
  {name: "Caroline", age: 21}
];
_.uniq(people, person => person.age);

Docs: http://underscorejs.org/#uniq

edited Apr 6, 2019 at 4:12

GammaGames

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answered Dec 8, 2013 at 2:37

jakecraige

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Comments

howrad · Accepted Answer · 2013-10-09 01:10:00Z

17

I think you're looking for the countBy function:

_UNIQUEAGEARRAY = _.countBy(_PERSONARRAY, "age");

It produces the result:

{"2":1,"7":2,"9":1}

JSFiddle demo: http://jsfiddle.net/4J2SX/

answered Oct 9, 2013 at 1:10

howrad

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2 Comments

Amitay Dobo Over a year ago

Then you can simply run it through map to get it in the object structure you wanted (updated at jsfiddle.net/4J2SX/2 )

Bhargava Over a year ago

@howard suppose I have this array of objects: [{couponid: 500 locationid: 10 },{couponid: 600 locationid: 15 }, { couponid: 500 locationid: 10 }, { couponid: 500 locationid: 20 } ] Using the solution given by you, I got an array of Unique coupon ids, with counts for each unique coupon id; it was the most concise anwer I found after lot of searhcing;Can you please tell, now how can i get, for each unique couponid, how many unique location ids ? ie in the above couponid 500 would have 2, and couponid 600 would have 1. Shall I post this as a separate question ?

Craig MacGregor · Accepted Answer · 2013-10-09 01:19:05Z

1

You can use underscore's groupBy if you want (might not be a good idea for a large dataset since it keeps a list of all the grouped items)

Example:

var d = _.groupBy(_PERSONARRAY, function(p){ 
    return p.age;
});

If you want to map this to your exact format try doing a map after the groupBy:

var x = _.map(d, function(people, age) {   
    return {uniqueAge: age,  numberOfPeople: people.length};   
});

jsFiddle: http://jsfiddle.net/jsgkC/2/

answered Oct 9, 2013 at 1:19

Craig MacGregor

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1 Comment

Craig MacGregor Over a year ago

Note: groupBy, countBy, indexBy all use the same "group" function internally so I don't know if one is that much better than the other from a performance point of view. You can see the annotated source here: underscorejs.org/docs/underscore.html

Mazz · Accepted Answer · 2017-05-10 12:22:07Z

0

groupBy function is useful.

_personsList = [{name:"anjo",age:5},{name:"george",age:3},{name:"jack",age:5}];
_uniqAgeList = _.groupBy(_personsList, "age");

will produce output

{
    "3":[{"name":"george","age":3}],
    "5":[{"name":"anjo","age":5},{"name":"jack","age":5}]
}

jsfiddle :http://jsfiddle.net/4J2SX/199/

edited May 10, 2017 at 12:22

Mazz

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answered May 10, 2017 at 11:01

Amil Sajeev

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