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.

Answer 1

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