Javascript count same array of object values based on two values

Question

Let's say we have an array of object like this

const studentRecord = [
 {
   name: 'John Doe',
   gender: 'Male',
   grade: '85'
 },{
   name: 'Steve Doe',
   gender: 'Male',
   grade: '85'
 },{
   name: 'Jane Doe',
   gender: 'Male',
   grade: '90'
 },{
   name: 'Mary Jane',
   gender: 'Female',
   grade: '85'
 },
]

I would like to count all students that are male that have the same grade and female students with the same grade something like this would be the output...

 85: {
       Male: 2,
       Female: 1,
     }
 90: {
       Female:1
     }

I only have this code so far

    const foo = {};
    studentRecord.reduce((prevVal, currentVal) => {
      foo[currentVal['gender']] = (foo[currentVal['gender']] || 0) + 1;
    });

Which only outputs {Male: 2, Female: 2}

CertainPerformance · Accepted Answer · 2020-02-19 02:46:29Z

When reducing into an object, create an object at the grade property first, if it doesn't exist. Then create an object at that grade's Male or Female property if it doesn't exist, and increment it:

const studentRecord = [
 {
   name: 'John Doe',
   gender: 'Male',
   grade: '85'
 },{
   name: 'Steve Doe',
   gender: 'Male',
   grade: '85'
 },{
   name: 'Jane Doe',
   gender: 'Male',
   grade: '90'
 },{
   name: 'Mary Jane',
   gender: 'Female',
   grade: '85'
 },
];

const results = studentRecord.reduce((a, { gender, grade }) => {
  if (!a[grade]) {
    a[grade] = {};
  }
  a[grade][gender] = (a[grade][gender] || 0) + 1;
  return a;
}, {});
console.log(results);

Or, without reduce:

const studentRecord = [
 {
   name: 'John Doe',
   gender: 'Male',
   grade: '85'
 },{
   name: 'Steve Doe',
   gender: 'Male',
   grade: '85'
 },{
   name: 'Jane Doe',
   gender: 'Male',
   grade: '90'
 },{
   name: 'Mary Jane',
   gender: 'Female',
   grade: '85'
 },
];

const results = {};
for (const { gender, grade } of studentRecord) {
  if (!results[grade]) {
    results[grade] = {};
  }
  results[grade][gender] = (results[grade][gender] || 0) + 1;
}
console.log(results);

thank you so much! worked like a charm. just a thought though can this be achieved dynamically? like you can choose which property in the object you would like to count
Sure, just make a function which takes the property to look up, and use bracket notation to look it up when accessing the record

ggorlen · Accepted Answer · 2020-02-19 02:45:40Z

Your code has the right idea regarding the gender key, but is missing the grade keys, which is necessary to produce the desired nested object.

The approach below checks whether a grade key exists yet and adds it if it doesn't. Then, it increments the appropriate gender for that key.

const studentRecords = [{ name: 'John Doe', gender: 'Male', grade: '85' }, { name: 'Steve Doe', gender: 'Male', grade: '85' }, { name: 'Jane Doe', gender: 'Male', grade: '90' }, { name: 'Mary Jane', gender: 'Female', grade: '85' }];

const byGrade = studentRecords.reduce((a, e) => {
  if (!(e.grade in a)) {
    a[e.grade] = {};
  }
  
  a[e.grade][e.gender] = ++a[e.grade][e.gender] || 1;
  return a;
}, {});

console.log(byGrade);

Sombriks · Accepted Answer · 2020-02-19 02:51:27Z

try this

const studentRecord = [
 {
   name: 'John Doe',
   gender: 'Male',
   grade: '85'
 },{
   name: 'Steve Doe',
   gender: 'Male',
   grade: '85'
 },{
   name: 'Jane Doe',
   gender: 'Male',
   grade: '90'
 },{
   name: 'Mary Jane',
   gender: 'Female',
   grade: '85'
 },
]

const grades = {};

studentRecord.map(e => { if(!grades[e.grade]) grades[e.grade] = {Male:0,Female:0}; });

studentRecord.map(e => grades[e.grade][e.gender]++ );

document.getElementById("result").innerHTML = JSON.stringify(grades)

<div id="result"></div>

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