How to calculate the average in JavaScript of the given properties in the array of objects

We have an array of objects. Each object contains a few properties and one of these properties is age:

const people = [
    {
        name: 'Anna',
        age: 22
    }, {
        name: 'Tom',
        age: 34
    }, {
        name: 'John',
        age: 12
    }, {
        name: 'Kallis',
        age: 22
    }, {
        name: 'Josh',
        age: 19
    }
]

We need to write a function that takes in such an array and returns the average of all the ages present in the array.

Method 1: Using reduce()

The most efficient approach is to use the reduce() method to sum all ages and divide by the array length:

const people = [
    {
        name: 'Anna',
        age: 22
    }, {
        name: 'Tom',
        age: 34
    }, {
        name: 'John',
        age: 12
    }, {
        name: 'Kallis',
        age: 22
    }, {
        name: 'Josh',
        age: 19
    }
]

const findAverageAge = (arr) => {
    const { length } = arr;
    return arr.reduce((acc, val) => {
        return acc + (val.age/length);
    }, 0);
};

console.log(findAverageAge(people));

21.8

Method 2: Sum First, Then Divide

An alternative approach is to first calculate the total sum, then divide by the length:

const people = [
    { name: 'Anna', age: 22 },
    { name: 'Tom', age: 34 },
    { name: 'John', age: 12 },
    { name: 'Kallis', age: 22 },
    { name: 'Josh', age: 19 }
];

const findAverageAge = (arr) => {
    const totalAge = arr.reduce((sum, person) => sum + person.age, 0);
    return totalAge / arr.length;
};

console.log(findAverageAge(people));

21.8

Method 3: Using map() and reduce()

You can also extract ages first using map(), then calculate the average:

const people = [
    { name: 'Anna', age: 22 },
    { name: 'Tom', age: 34 },
    { name: 'John', age: 12 },
    { name: 'Kallis', age: 22 },
    { name: 'Josh', age: 19 }
];

const findAverageAge = (arr) => {
    const ages = arr.map(person => person.age);
    const sum = ages.reduce((acc, age) => acc + age, 0);
    return sum / ages.length;
};

console.log(findAverageAge(people));

21.8

Generalized Function for Any Property

Here's a reusable function that can calculate the average of any numeric property:

const data = [
    { name: 'Anna', age: 22, salary: 50000 },
    { name: 'Tom', age: 34, salary: 75000 },
    { name: 'John', age: 12, salary: 0 }
];

const findAverage = (arr, property) => {
    const sum = arr.reduce((acc, obj) => acc + obj[property], 0);
    return sum / arr.length;
};

console.log("Average age:", findAverage(data, 'age'));
console.log("Average salary:", findAverage(data, 'salary'));

Average age: 22.666666666666668
Average salary: 41666.666666666664

Comparison

Conclusion

The most efficient approach is to sum all values using reduce() and then divide by the array length. For reusability, create a generalized function that accepts the property name as a parameter.