Filter unique array values and sum in JavaScript

In JavaScript, you may need to filter arrays that contain duplicate values and combine their numeric values. This is common when processing data like product orders, transactions, or inventory records.

Problem Statement

Given an array of arrays where each subarray has three elements [id, name, amount], we need to:

• Remove duplicate entries based on the first element (id)
• Sum the third element (amount) for matching entries

const arr = [[12345, "product", "10"],[12345, "product", "15"],[1234567, "other", "10"]];

The expected output should combine the duplicate entries and sum their amounts:

[[12345, "product", "25"], [1234567, "other", "10"]]

Solution Using Map for Tracking

const arr = [[12345, "product", "10"],[12345, "product", "15"],[1234567, "other", "10"]];

const addSimilar = (arr = []) => {
    const res = [];
    const map = {};
    
    arr.forEach(el => {
        const [id, name, amount] = el;
        
        if(map.hasOwnProperty(id)){
            // If ID exists, add amounts and update existing entry
            const newAmount = +amount + +res[map[id] - 1][2];
            res[map[id] - 1][2] = '' + newAmount;
        } else {
            // If new ID, add to result and track position
            map[id] = res.push(el);
        }
    });
    
    return res;
};

console.log(addSimilar(arr));

[ [ 12345, 'product', '25' ], [ 1234567, 'other', '10' ] ]

How It Works

The algorithm uses a map object to track which IDs have been processed:

1. First occurrence: Store the entry in result array and record its position in the map
2. Duplicate found: Add the amount to the existing entry's amount
3. Type conversion: Use +amount to convert strings to numbers for addition
4. Result format: Convert sum back to string to maintain original data type

Alternative Approach Using reduce()

const arr = [[12345, "product", "10"],[12345, "product", "15"],[1234567, "other", "10"]];

const addSimilarReduce = (arr = []) => {
    const grouped = arr.reduce((acc, [id, name, amount]) => {
        if (acc[id]) {
            acc[id][2] = '' + (+acc[id][2] + +amount);
        } else {
            acc[id] = [id, name, amount];
        }
        return acc;
    }, {});
    
    return Object.values(grouped);
};

console.log(addSimilarReduce(arr));

[ [ 12345, 'product', '25' ], [ 1234567, 'other', '10' ] ]

Comparison

Conclusion

Both approaches effectively filter duplicates and sum values. The reduce method is more concise, while the map tracking approach offers more explicit control over the result structure.