Find the number of times a value of an object property occurs in an array with JavaScript?

When working with arrays of objects, you often need to count how many times a specific property value appears. JavaScript's reduce() method combined with Map provides an efficient solution for this task.

Using reduce() with Map

The reduce() method processes each array element to build a frequency count. We use a Map to store the counts because it maintains insertion order and provides efficient lookups.

const subjectDetails = [
    {
        subjectId: '101',
        subjectName: 'JavaScript'
    },
    {
        subjectId: '102',
        subjectName: 'Java'
    },
    {
        subjectId: '103',
        subjectName: 'JavaScript'
    },
    {
        subjectId: '104',
        subjectName: 'Python'
    },
    {
        subjectId: '105',
        subjectName: 'JavaScript'
    }
];

const frequencyCount = [...subjectDetails.reduce((obj1, obj2) => {
    if (obj1.has(obj2.subjectName)) {
        obj1.get(obj2.subjectName).frequency++;
    } else {
        obj1.set(obj2.subjectName, { 
            subjectName: obj2.subjectName,
            frequency: 1 
        });
    }
    return obj1;
}, new Map()).values()];

console.log(frequencyCount);

[
  { subjectName: 'JavaScript', frequency: 3 },
  { subjectName: 'Java', frequency: 1 },
  { subjectName: 'Python', frequency: 1 }
]

How It Works

The algorithm follows these steps:

1. reduce() iterates through each object in the array
2. For each object, it checks if the property value already exists in the Map
3. If it exists, increment the frequency counter
4. If it doesn't exist, create a new entry with frequency 1
5. Convert Map values back to an array using spread operator

Alternative: Using forEach with Object

You can also use a plain object instead of Map for simpler syntax:

const subjectDetails = [
    { subjectId: '101', subjectName: 'JavaScript' },
    { subjectId: '102', subjectName: 'Java' },
    { subjectId: '103', subjectName: 'JavaScript' },
    { subjectId: '104', subjectName: 'Python' }
];

const frequencyObj = {};

subjectDetails.forEach(subject => {
    if (frequencyObj[subject.subjectName]) {
        frequencyObj[subject.subjectName]++;
    } else {
        frequencyObj[subject.subjectName] = 1;
    }
});

console.log(frequencyObj);

{ JavaScript: 2, Java: 1, Python: 1 }

Counting Specific Property Values

To count occurrences of any object property, create a reusable function:

function countPropertyValues(array, propertyName) {
    return array.reduce((counts, obj) => {
        const value = obj[propertyName];
        counts[value] = (counts[value] || 0) + 1;
        return counts;
    }, {});
}

const students = [
    { name: 'Alice', grade: 'A' },
    { name: 'Bob', grade: 'B' },
    { name: 'Charlie', grade: 'A' },
    { name: 'David', grade: 'C' }
];

console.log(countPropertyValues(students, 'grade'));

{ A: 2, B: 1, C: 1 }

Conclusion

Use reduce() with Map for complex frequency counting with objects, or simple object accumulation for basic counting. Both approaches efficiently count property value occurrences in object arrays.