How to sum all elements in a nested array? JavaScript

Let's say, we are supposed to write a function that takes in a nested array of Numbers and returns the sum of all the numbers. We are required to do this without using the Array.prototype.flat() method.

Let's write the code for this function ?

Using Recursive Approach

const arr = [
    5,
    7,
    [ 4, [2], 8, [1,3], 2 ],
    [ 9, [] ],
    1,
    8
];

const findNestedSum = (arr) => {
    let sum = 0;
    for(let len = 0; len < arr.length; len++){
        sum += Array.isArray(arr[len]) ? findNestedSum(arr[len]) : arr[len];
    }
    return sum;
};

console.log(findNestedSum(arr));

50

How It Works

The function uses recursion to handle nested arrays:

• For each element, it checks if it's an array using Array.isArray()
• If it's an array, it recursively calls itself to sum that sub-array
• If it's a number, it adds it directly to the sum
• Empty arrays contribute 0 to the final sum

Alternative Using reduce()

const findNestedSumReduce = (arr) => {
    return arr.reduce((sum, item) => {
        return sum + (Array.isArray(item) ? findNestedSumReduce(item) : item);
    }, 0);
};

const testArr = [1, [2, [3, 4], 5], 6, [7]];
console.log(findNestedSumReduce(testArr));

28

Comparison

Conclusion

Both approaches effectively sum nested arrays using recursion. The for-loop version is slightly more performant, while the reduce() method offers cleaner functional programming style.