Sum of array object property values in new array of objects in Javascript [duplicate]

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Simple reduce solution :

const data = [
  { 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' }];

const result = data.reduce((cur, val) => {
  let alreadyIn = cur.find(e => e['subject'] == val['subject']);
  if (alreadyIn) {
    alreadyIn['marks'] = (parseInt(alreadyIn['marks']) + parseInt(val['marks'])).toString();
    alreadyIn['noOfStudents'] = (parseInt(alreadyIn['noOfStudents']) + parseInt(val['noOfStudents'])).toString();
  } else {
    cur.push(val);
  }
  return cur;
}, []);

console.log(result);

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