Sum all duplicate values in array in JavaScript

We need to write a JavaScript function that takes an array of numbers with duplicate entries and sums all duplicate values. Each unique number appears once in the result, multiplied by its frequency count.

Problem Understanding

For array [1, 3, 1, 3, 5, 7, 5, 3, 4]:

• 1 appears 2 times ? 1 × 2 = 2
• 3 appears 3 times ? 3 × 3 = 9
• 5 appears 2 times ? 5 × 2 = 10
• 7 appears 1 time ? 7 × 1 = 7
• 4 appears 1 time ? 4 × 1 = 4

Using Map to Count and Sum

This approach uses a Map to count occurrences, then multiplies each unique value by its count:

const input = [1, 3, 1, 3, 5, 7, 5, 3, 4];

const sumDuplicate = arr => {
    const map = arr.reduce((acc, val) => {
        if(acc.has(val)){
            acc.set(val, acc.get(val) + 1);
        }else{
            acc.set(val, 1);
        };
        return acc;
    }, new Map());
    return Array.from(map, el => el[0] * el[1]);
};

console.log(sumDuplicate(input));

[ 2, 9, 10, 7, 4 ]

Using Object for Counting

Alternative approach using a plain object instead of Map:

const input = [1, 3, 1, 3, 5, 7, 5, 3, 4];

const sumDuplicateWithObject = arr => {
    const count = {};
    
    // Count occurrences
    arr.forEach(num => {
        count[num] = (count[num] || 0) + 1;
    });
    
    // Multiply each number by its count
    return Object.keys(count).map(key => parseInt(key) * count[key]);
};

console.log(sumDuplicateWithObject(input));

[ 2, 9, 10, 7, 4 ]

Using forEach for Direct Calculation

More readable approach that separates counting and multiplication:

const input = [1, 3, 1, 3, 5, 7, 5, 3, 4];

const sumDuplicateSimple = arr => {
    const frequency = new Map();
    
    // Count frequencies
    arr.forEach(num => {
        frequency.set(num, (frequency.get(num) || 0) + 1);
    });
    
    // Calculate sums
    const result = [];
    frequency.forEach((count, value) => {
        result.push(value * count);
    });
    
    return result;
};

console.log(sumDuplicateSimple(input));

[ 2, 9, 10, 7, 4 ]

Comparison

Conclusion

All methods count duplicate occurrences and multiply each unique value by its frequency. Choose based on readability preference and team coding standards.