Today’s problem of this DevAdvent 2022 is really simple. I could categorize it among the essential things to know in JavaScript: how to convert a number to a string. But although it is a simple problem, there are various solutions. Let’s see what they are.

The problem

link to the Kata

We need a function that can transform a number (integer) into a string. What ways of achieving this do you know?


123  --> "123"
999  --> "999"
-100 --> "-100"

My solution: template literals

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The first solution that comes to mind is to use template literals. Mozilla defines them like this:

Template literals are literals delimited with backtick (`) characters, allowing for multi-line strings, string interpolation with embedded expressions, and special constructs called tagged templates.

Consequently this is my solution:

const numberToString = (x) => `${x}`;


But that’s not the only way forward. One solution is to use the Number.toString() method. In some cases it is even the best solution.

const numberToString = (x) => x.toString();

This method takes one parameter, called radix. The radix is an integer in the range 2 through 36 specifying the base to use for representing the number value. Defaults to 10.

Basically it allows you to convert a number from one base to another and get a string with the result. The only thing to remember is that to use it directly on a number (and not on a variable) you need to surround the number itself with tone brackets.

10.toString(); // SyntaxError: Invalid or unexpected token

(10).toString(); // "10"
(10).toString(2); // "1010"
(10).toString(8); // "12"
(10).toString(16); // "a"


Another way to convert a number to string in JavaScript can be using the String() constructor. The String constructor is used to create a new String object. When called instead as a function, it performs type conversion to a primitive string.

const numberToString = (x) => x.toString();

But if we decide to go this route we can simplify the code even more:

const numberToString = String;

In doing so I combine the numberToString constant with the String() constructor. So writing numberToString(10) is the same as writing String(10). It’s a matter of taste.

String concatenation

Another possible variation is to use string concatenation. This forces JS to convert the number to a string automatically. It’s a variation of my solution:

const numberToString = (x) => "" + x;

Custom toString()

The last solution is the most complicated. But certainly the most intriguing. It’s about reproducing the toString() method, to analyze its algorithm. This code aims to do what toString() does (for integers), but without using the built-in function.

Not my code, but Xormias’.

function numberToString(num) {
  //create a new empty string in which the result will be written
  var str = "";

  //Check, whether the number is positive or negative. The multiplier 1 or -1 will be saved for later
  const mult = num < 0 ? -1 : 1;

  //Take the absolut value of num. Could also be done with Math.abs
  num *= mult;

  //Loop, which will run as often as num has digits
  do {
    //Take the least significant digit of num (num % 10), add 48 and get the associated ascii character. The number 0 has the ascii value of 48, 1 = 49... 9 = 57.
    //Then prepend that character to the string.
    str = String.fromCharCode((num % 10) + 48) + str;

    //Remove the least significant digit from num. (Divide by ten, then round down)
    num = ~~(num / 10);
  } while (num > 0);

  //If the input number was negative, prepend a -.
  if (mult < 0) str = "-" + str;
  return str;

I’ve come up with 5 ways to convert a number to string. From the simplest to the most complex. It wasn’t such a trivial problem, in the end.