Digitize a number in JavaScript

Converting a number to an array of digits is fairly easy in JavaScript. All it requires is some string manipulation and array methods.

Digitize a number

For starters, any number can be converted to a string using the template literal syntax. Converting a string to an array of characters is as simple as using the spread operator (...). Then, to convert a character into a number, you can use Number.parseInt(). Finally, to combine these two steps, you can use Array.prototype.map() to transform each character into a number.

As an additional step, we can use Math.abs() to remove the sign of the number before converting it to a string. This way, we can handle both positive and negative numbers.

const digitize = n =>
  [...`${Math.abs(n)}`].map(i => Number.parseInt(i, 10));

digitize(123); // [1, 2, 3]
digitize(-123); // [1, 2, 3]

Sum of digits

If you want to sum the digits of a number, you can use Array.prototype.reduce() on the array of digits. This will allow you to accumulate the sum of all digits in a single pass.

const sumDigits = n =>
  digitize(n).reduce((acc, curr) => acc + curr, 0);

sumDigits(123); // 6
sumDigits(-123); // 6

Digital root

The digital root of a number is the single-digit value obtained by an iterative process of summing digits, until a single-digit number is achieved. You can compute the digital root using the sumDigits function in a loop until the result is a single digit.

const digitalRoot = n => {
  let sum = sumDigits(n);
  while (sum > 9) sum = sumDigits(sum);
  return sum;
};

digitalRoot(123); // 6
digitalRoot(-123); // 6

However, a more efficient way to compute the digital root is to use the modulo (%) operator in conjuction with the number 9. The digital root can be derived from the properties of numbers in base 10, where the digital root is equivalent to the number modulo 9. This method is particularly useful because it avoids the need for iterative summation.

const digitalRoot = n => {
  if (n === 0) return 0;
  if (n % 9 != 0) return n % 9;
  return 9;
};

digitalRoot(123); // 6
digitalRoot(-123); // 6

Finally, another similarly efficient way to compute the digital root is to use the formula 1 + (n - 1) % 9, which works for all integers except zero.

const digitalRoot = n =>
  n === 0 ? 0 : 1 + (Math.abs(n) - 1) % 9;

digitalRoot(123); // 6
digitalRoot(-123); // 6

Digitize a number in JavaScript