How to count number of objects in array by property? [duplicate]

Question

I'm trying to make a simple "top 3" of the users that have the most messages. So far I was able to group the user and messages inside an array of objects

let data = [ { message: 'fsaasfafs', user: 'John' },
  { message: 'asgsgaasgags', user: 'John' },
  { message: 'asgsgaasgags', user: 'John' },
  { message: 'asgsgaasgags', user: 'Smith' },
  { message: 'asgsgaasgags', user: 'Samantha' },
  { message: 'asgsgaasgags', user: 'Luis' },
  { message: 'asgsgaasgags', user: 'Samantha' }]

console.log(data);

In this case, the top 3 would be John - 3 ; Samantha - 2; Smith or Luis (does not matter) - 1 I would like a map with the user as key and the amount of messages.

Any help please?

Please post what you have tried to achieve the desired result. — Krishna Prashatt
– Krishna Prashatt, Commented Nov 7, 2019 at 11:01
@jonas - How does sorting answer this question? (I've added a dupe that I think does, I'm just curious about the one you picked.) — T.J. Crowder
– T.J. Crowder, Commented Nov 7, 2019 at 11:02
@t.j. well, I was just about to do the same (adding a few more duplicates), but you two were faster :) — Jonas Wilms
– Jonas Wilms, Commented Nov 7, 2019 at 11:03
let statsMap = {}; data.forEach(message => { if(statsMap.hasProperty(message.user)) { } else { statsMap[message.user] = } }); i got stuck — Tiago
– Tiago, Commented Nov 7, 2019 at 11:05

Saurabh Yadav · Accepted Answer · 2019-11-08 09:09:23Z

1

    let data = [ { message: 'fsaasfafs', user: 'John' },
      { message: 'asgsgaasgags', user: 'John' },
      { message: 'asgsgaasgags', user: 'John' },
      { message: 'asgsgaasgags', user: 'Smith' },
      { message: 'asgsgaasgags', user: 'Samantha' },
      { message: 'asgsgaasgags', user: 'Luis' },
      { message: 'asgsgaasgags', user: 'Samantha' }];
      
    let topM = Object.entries(data.reduce((acc, c) => { acc[c.user] = (acc[c.user] || 0 ) + 1; return acc;},{})).sort(function (a, b) { return b[1] - a[1];}).slice(0, 3);
    console.log(topM);

Hope This helps you !

edited Nov 8, 2019 at 9:09

answered Nov 7, 2019 at 11:08

Saurabh Yadav

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Comments

prasanth · Accepted Answer · 2019-11-07 11:24:23Z

0

You could do like get count of each user message

Then you could count the each user messages using Array#reduce with Array#filter(this will match the user count in array for each user)

let data = [ { message: 'fsaasfafs', user: 'John' },
  { message: 'asgsgaasgags', user: 'John' },
  { message: 'asgsgaasgags', user: 'John' },
  { message: 'asgsgaasgags', user: 'Smith' },
  { message: 'asgsgaasgags', user: 'Samantha' },
  { message: 'asgsgaasgags', user: 'Luis' },
  { message: 'asgsgaasgags', user: 'Samantha' }]
  
 let user_list = data.reduce((a,b,c,d)=>{
   if(Object.keys(a).indexOf(b.user) == -1){
    a[b.user] = d.filter(i=>i.user == b.user).length
   }
   return a
 },{})
 
 console.log(user_list)

edited Nov 7, 2019 at 11:24

answered Nov 7, 2019 at 11:07

prasanth

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3 Comments

Tiago Over a year ago

please explain :/

trincot Over a year ago

This is not very efficient. Not only does it involve three iterations (map, filter, and reduce) when it could be done in one, it also has a nested iteration happening with indexOf, which gives it a O(n²) time complexity. On top of that, this question has been asked before and has useful answers already.

prasanth Over a year ago

@trincot i just update method with single parent loop

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