Sum of array object property values in new array of objects in Javascript

Question

I have an array of objects where i need sum of object property values in new array of objects,

Input:

var inputArray = [
  { subject: \'Maths\', mark         


        

Answer 1

You can use forEach() to iterate and generate the new array

var inputArray = [
  { subject: 'Maths', marks: '40', noOfStudents: '5' },
  { subject: 'Science', marks: '50', noOfStudents: '16' },
  { subject: 'History', marks: '35', noOfStudents: '23' },
  { subject: 'Science', marks: '65', noOfStudents: '2' },
  { subject: 'Maths', marks: '30', noOfStudents: '12' },
  { subject: 'History', marks: '55', noOfStudents: '20' }
],
    res = [],
    key = {};

inputArray.forEach(function(v) {
  if (key.hasOwnProperty(v.subject)) { // check subject already added by using key object
    res[key[v.subject]].marks += Number(v.marks); //incase already exist parse number and add
    res[key[v.subject]].noOfStudents += Number(v.noOfStudents);
  } else {
    key[v.subject] = res.length; // create index entry in key object
    res.push({ // push the value 
      'subject': v.subject,
      'marks': Number(v.marks),
      'noOfStudents': Number(v.noOfStudents)
    })
    // if you pushed the original object then the original array also will get updated while adding the mark, so never push the refernce
  }
})

console.log(res);

---

Using reduce() method

var inputArray = [
  { subject: 'Maths', marks: '40', noOfStudents: '5' },
  { subject: 'Science', marks: '50', noOfStudents: '16' },
  { subject: 'History', marks: '35', noOfStudents: '23' },
  { subject: 'Science', marks: '65', noOfStudents: '2' },
  { subject: 'Maths', marks: '30', noOfStudents: '12' },
  { subject: 'History', marks: '55', noOfStudents: '20' }
],
    key = {};

res=inputArray.reduce(function(arr,v) {
  if (key.hasOwnProperty(v.subject)) { // check subject already added by using key object
    arr[key[v.subject]].marks += Number(v.marks); //incase already exist parse number and add
    arr[key[v.subject]].noOfStudents += Number(v.noOfStudents);
  } else {
    key[v.subject] = arr.length; // create index entry in key object
    arr.push({ // push the value 
      'subject': v.subject,
      'marks': Number(v.marks),
      'noOfStudents': Number(v.noOfStudents)
    })
    // if you pushed the original object then the original array also will get updated while adding the mark, so never push the refernce     
  }
  return arr;
},[])

console.log(res);

---

FYI : You can avoid the key object by using find() method, but performance wise that may be little bit slower.