How to use javascript (filter) to count the frequency of an object value?

Question

This is the url from which i have to fetch data.I want the frequency of the postIds.How can i do this using the methods (map,filter or reduce).I've done it using a loop .Can it be done in a better way?.please help..

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
  fetch('http://jsonplaceholder.typicode.com/comments')
  .then(
    function(response) {
      if (response.status !== 200) {
        console.log('Looks like there was a problem. Status Code: ' +
          response.status);
        return;
      }
      response.json().then(function(data) 
      {
          var na=[];
          for(var i=1;i<=100;i++)
          {
            var a= data.filter(ab=> {
          return ab.postId==i;});
         // console.log(a); 
          na.push({PostId:i,frequency:a.length});
          }
          console.log(na);
      }
  )})
  .catch(function(err) {
    console.log('Fetch Error :-S', err);
  });
    </script>
</body>
</html>

Note the answers given only iterate the array once whereas using filter() to create a new array to get it's length iterates the array once for every item in it — charlietfl
– charlietfl, Commented Jun 28, 2020 at 20:03
@-charlietfl thank you...i knew there must be a better way to do this..what i was doing was similar to using a nested loop.. — Shivam_Yadav
– Shivam_Yadav, Commented Jun 28, 2020 at 20:15
Right...plus each returned filter array takes up needless memory until it gets garbage collected — charlietfl
– charlietfl, Commented Jun 28, 2020 at 20:20
@charlietfl yeah ..first i thought of emptying the array with a=[] but i knew i had to post it as a question so i shouldn't optimize a wrong approach lol..and i used a predefined length as well which was also wrong..thank you once again — Shivam_Yadav
– Shivam_Yadav, Commented Jun 28, 2020 at 20:24

sonEtLumiere · Accepted Answer · 2022-07-23 00:42:52Z

2

I hope this helps

let counterObj = {};

let cars = [
    { id: 1, name: 'Mercedes', year: '2015' },
    { id: 2, name: 'Mercedes', year: '2000' },
    { id: 3, name: 'BMW', year: '2010' },
    { id: 4, name: 'BMW', year: '2004' },
    { id: 5, name: 'Volvo', year: '2012' },
    { id: 6, name: 'Volvo', year: '2014' } 
];


for (let item of cars){
    counterObj[item.name] = 1 + (counterObj[item.name] || 0)
}

console.log(counterObj);

edited Jul 23, 2022 at 0:42

answered Jun 28, 2020 at 19:42

sonEtLumiere

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1 Comment

Shivam_Yadav Over a year ago

I have to solve it with reduce,map or filter and nothing else.Thank you for answering but i want a solution that uses these methods only.

slumbergeist · Accepted Answer · 2020-06-28 19:58:18Z

1

You can use reduce to generate a map of PostId with its frequency.

function mapFrequency(data) {
     return data.reduce((map, datum) => {
        if (map[datum.postId]) {
          map[datum.postId] += 1;
        } else {
          map[datum.postId] = 1 
        }
        return map;
        
     }, {})
}

This function will create an object with keys as postId and value as its frequency.

If you want to generate an array as in your sample, you can then do

 const frequencies = mapFrequency(data);
 const result = Object.keys(frequencies).map((id) => {
   return {
      PostId: id, 
      frequency: frequencies[id]
   }
  });

answered Jun 28, 2020 at 19:58

slumbergeist

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1 Comment

Shivam_Yadav Over a year ago

Thank You for your help : )

Konstantin · Accepted Answer · 2020-06-28 19:49:23Z

1

With reduce you can do something like this:

const na = data.reduce((acc, el) => {
  acc[el.postId] = acc[el.postId] ? acc[el.postId] + 1 : 1;
  return acc;
}, {});

Pretty much the same as @sonEtLumiere suggested, but with reduce

answered Jun 28, 2020 at 19:49

Konstantin

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1 Comment

Shivam_Yadav Over a year ago

thank you..this is how i wanted ..I just wanted to know that can I use map on your answer to get the result in the form of {postId: postIdNumber,frequency: frequencyOfPostId}?

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