Find Sum of Digits in JavaScript

What You’ll Learn

Digit sum means adding every digit of a number: 12345 → 1+2+3+4+5 = 15. Extract digits with % 10 and Math.floor(n / 10), use Math.abs() for negatives, and stop when the value becomes 0. This tutorial covers a live preview, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given an integer n, return the sum of its decimal digits (sign ignored).

# 12345 -> 1+2+3+4+5 = 15
# -802  -> 8+0+2     = 10
# 0     -> 0

Inputs & Outputs

Minimal workflow

function digitSum(n) {
  n = Math.abs(n);
  let total = 0;
  while (n > 0) {
    total += n % 10;
    n = Math.floor(n / 10);
  }
  return total;
}

Method comparison

🔮 Live Preview

Enter any integer (e.g. 12345 or -802) and see the digits plus their sum.

Press “Compute”.

Examples Gallery

Three complete JavaScript programs — sum digits of 12345, read user input, and trace peeling for a negative value. Click View Output to reveal sample console results.

function digitSum(number) {
  number = Math.abs(number);
  let total = 0;
  while (number !== 0) {
    total += number % 10;
    number = Math.floor(number / 10);
  }
  return total;
}

const number = 12345;
console.log(`The sum of digits of ${number} is: ${digitSum(number)}`);

The sum of digits of 12345 is: 15

function digitSum(number) {
  number = Math.abs(number);
  let total = 0;
  while (number !== 0) {
    total += number % 10;
    number = Math.floor(number / 10);
  }
  return total;
}

// Simulated input (edit this value)
const number = 12345;
console.log(`Sum of digits: ${digitSum(number)}`);

Enter a number: 12345
Sum of digits: 15

const n = -802;
let x = Math.abs(n);
let total = 0;
console.log(`Starting from Math.abs(${n}) = ${x}`);
while (x !== 0) {
  const digit = x % 10;
  total += digit;
  console.log(`  take ${digit}, remaining ${Math.floor(x / 10)}, total=${total}`);
  x = Math.floor(x / 10);
}
console.log(`Digit sum: ${total}`);

Starting from Math.abs(-802) = 802
  take 2, remaining 80, total=2
  take 0, remaining 8, total=2
  take 8, remaining 0, total=10
Digit sum: 10

🔎 Worked Walkthrough — 12345

Watch digits peel from the right while the accumulator climbs to 15.

Same steps for -802 after abs: 2, then 0, then 8 → total 10.

⚖️ Facts Worth Knowing

Handy follow-ups interviewers sometimes ask.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

d is the number of digits — about floor(log₁₀ |n|) + 1 for n ≠ 0.

💡 Best Practices