create unique list and count of array items in array of objects

Question

I have the following source array:

const list = [
    {
        students: [ 'peter', 'bob', 'john']
    },
    {
        students: [ 'thomas', 'sarah', 'john']
    },
    {
        students: [ 'john', 'sarah', 'jack']
    }
];

and i want to get the unique student names and their count, final result should be like:

{
    'john': 3,
    'sarah': 2,
    'thomas': 1,
    'jack': 1,
    'peter': 1,
    'bob': 1
}

here is my attempt:

const unique = list.reduce(function(total, curr){
  const students = curr.students;
  for (c of students) {
    if (!total[c]) {
      total[c] = 1
    } else {
      total[c] += 1;
    }
  }
  return total;

}, {});

is there a better way to do it? or faster and cleaner way? thanks

Your expected output has the most frequently occurring names first in the object - is that required? (it's possible to achieve, just kinda odd to be a requirement, since a good script probably shouldn't depend on property order) — CertainPerformance
– CertainPerformance, Commented Sep 24, 2019 at 23:58

CertainPerformance · Accepted Answer · 2019-09-25 00:08:20Z

2

I'd flatten the arrays first, then count up with reduce:

const list = [
    {
        students: [ 'peter', 'bob', 'john']
    },
    {
        students: [ 'thomas', 'sarah', 'john']
    },
    {
        students: [ 'john', 'sarah', 'jack']
    }
];


const allStudents = list.flatMap(({ students }) => students);
const count = allStudents.reduce((a, name) => {
  a[name] = (a[name] || 0) + 1;
  return a;
}, {});
console.log(count);

If you want the properties to be ordered as well, then take the Object.entries of the object, sort it, then turn it back into an object with Object.fromEntries:

const list = [
    {
        students: [ 'peter', 'bob', 'john']
    },
    {
        students: [ 'thomas', 'sarah', 'john']
    },
    {
        students: [ 'john', 'sarah', 'jack']
    }
];


const allStudents = list.flatMap(({ students }) => students);
const count = allStudents.reduce((a, name) => {
  a[name] = (a[name] || 0) + 1;
  return a;
}, {});
const sorted = Object.fromEntries(
  Object.entries(count).sort((a, b) => b[1] - a[1])
);
console.log(sorted);

If your environment doesn't support flatMap, or fromEntries, use a polyfill, or flatten/group with a different method:

const list = [
    {
        students: [ 'peter', 'bob', 'john']
    },
    {
        students: [ 'thomas', 'sarah', 'john']
    },
    {
        students: [ 'john', 'sarah', 'jack']
    }
];


const allStudents = [].concat(...list.map(({ students }) => students));
const count = allStudents.reduce((a, name) => {
  a[name] = (a[name] || 0) + 1;
  return a;
}, {});
const sortedEntries = Object.entries(count).sort((a, b) => b[1] - a[1]);
const sortedObj = sortedEntries.reduce((a, [prop, val]) => {
  a[prop] = val;
  return a;
}, {});
console.log(sortedObj);

Keep in mind that object property order is only specified in ES6+ environments. While Object.fromEntries isn't guaranteed by the specification to create an object in the same order as the entries, it does anyway, in any implementation I've ever encountered, luckily. (If you're still worried about it, you can use the old-fashioned reduce method to create the object instead, like in the third snippet)

edited Sep 25, 2019 at 0:08

answered Sep 24, 2019 at 23:56

CertainPerformance

373k55 gold badges354 silver badges359 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

teebeetee Over a year ago

TypeError: list.flatMap is not a function

ggorlen Over a year ago

Change it to [].concat(...list.map(({ students }) => students)) or list.map(({ students }) => students).flat().

Jaromanda X Over a year ago

@storis if list.flatMap is not a function, what browser or version of nodejs are you running?

sinnerwithguts · Accepted Answer · 2019-09-25 13:41:49Z

0

Try using functional programming: combination of map and 2 reduce methods.

const listMapped = list.map(it=> it.students)

const listReduced = listMapped.reduce((acc, rec) => {
return [...acc.concat(rec)]
}, [])

const listCounted = listReduced.reduce((acc, rec) => {
acc[rec]
? acc[rec] += 1
: acc[rec] = 1
return acc 
}, {})

console.log(listCounted)

answered Sep 25, 2019 at 13:41

sinnerwithguts

69310 silver badges22 bronze badges

Collectives™ on Stack Overflow

create unique list and count of array items in array of objects

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related