Negative number digit sum in JavaScript

We are required to write a JavaScript function that takes in a negative integer and returns the sum of its digits.

Example Input

For example: If the number is:

-5456

Expected Output

Then the output should be:

5+4+5+6 = 20

Using String Manipulation and Reduce

The following approach converts the number to string, splits it into digits, and uses reduce to sum them:

const num = -5456;
const sumNum = num => {
    return String(num).split("").reduce((acc, val, ind) => {
        if(ind === 0){
            return acc;
        }
        if(ind === 1){
            acc -= +val;
            return acc;
        };
        acc += +val;
        return acc;
    }, 0);
};
console.log(sumNum(num));

20

Simpler Approach Using Math.abs()

A cleaner solution uses Math.abs() to remove the negative sign and then sum the digits:

const digitSum = (num) => {
    return Math.abs(num)
        .toString()
        .split('')
        .reduce((sum, digit) => sum + parseInt(digit), 0);
};

console.log(digitSum(-5456)); // 20
console.log(digitSum(-123));  // 6
console.log(digitSum(-999));  // 27

20
6
27

Using While Loop

An alternative mathematical approach without string conversion:

const digitSumLoop = (num) => {
    num = Math.abs(num); // Remove negative sign
    let sum = 0;
    
    while (num > 0) {
        sum += num % 10;   // Get last digit
        num = Math.floor(num / 10); // Remove last digit
    }
    
    return sum;
};

console.log(digitSumLoop(-5456)); // 20
console.log(digitSumLoop(-789));  // 24

20
24

Comparison

Conclusion

Use Math.abs() with string methods for simplicity and readability. The while loop approach is most efficient for performance-critical applications.