Digit sum upto a number of digits of a number in JavaScript

We are required to write a JavaScript function that takes in two numbers, let's say m and n as arguments.

n will always be smaller than or equal to the number of digits present in m. The function should calculate and return the sum of first n digits of m.

Problem Statement

If the input numbers are:

const m = 5465767;
const n = 4;

Then the output should be:

20

because 5 + 4 + 6 + 5 = 20

Solution Using String Conversion

The most straightforward approach is to convert the number to a string and iterate through the first n characters:

const m = 5465767;
const n = 4;

const digitSumUpto = (m, n) => {
    if(n > String(m).length){
        return 0;
    };
    let sum = 0;
    for(let i = 0; i < n; i++){
        const el = +String(m)[i];
        sum += el;
    };
    return sum;
};

console.log(digitSumUpto(m, n));

20

Alternative Solution Using Mathematical Approach

We can also solve this without string conversion by extracting digits mathematically:

const digitSumMath = (m, n) => {
    const numStr = String(m);
    if(n > numStr.length) return 0;
    
    let sum = 0;
    let temp = m;
    const totalDigits = numStr.length;
    
    // Extract digits from left to right
    for(let i = 0; i < n; i++){
        const divisor = Math.pow(10, totalDigits - i - 1);
        const digit = Math.floor(temp / divisor);
        sum += digit;
        temp = temp % divisor;
    }
    
    return sum;
};

console.log(digitSumMath(5465767, 4)); // 20
console.log(digitSumMath(123456, 3));  // 6 (1+2+3)

20
6

How It Works

The string conversion method works by:

1. Converting the number to a string to easily access individual digits
2. Iterating through the first n characters
3. Converting each character back to a number using the unary plus operator (+)
4. Adding each digit to the running sum

Edge Cases

// Test edge cases
console.log(digitSumUpto(123, 5));    // 0 (n > number of digits)
console.log(digitSumUpto(987654, 0)); // 0 (no digits to sum)
console.log(digitSumUpto(5, 1));      // 5 (single digit)

0
0
5

Conclusion

The string conversion approach is simpler and more readable for digit manipulation. Use this method when you need to sum the first n digits of any number efficiently.