JavaScript Program to Reverse a Number with Output

You can reverse a number in JavaScript by converting it to a string, splitting it, reversing the array, joining it back, and converting to a number using Number(String(num).split('').reverse().join('')).

Examples

Input12345

Output54321

Input1000

Output1

Input0

Output0

How to Think About It

To reverse a number, think of it like reversing a word: first change the number into text so you can handle each digit separately, then flip the order of these digits, and finally turn it back into a number to get the reversed value.

Algorithm

1

Get the input number.

2

Convert the number to a string to access each digit.

3

Split the string into an array of characters.

4

Reverse the array to flip the digits.

5

Join the reversed array back into a string.

6

Convert the string back to a number and return it.

Code

javascript

function reverseNumber(num) {
  return Number(String(num).split('').reverse().join(''));
}

console.log(reverseNumber(12345));

Output

54321

Dry Run

Let's trace reversing the number 12345 through the code

1

Convert number to string

num = 12345 -> String(num) = '12345'

2

Split string into array

'12345' -> ['1', '2', '3', '4', '5']

3

Reverse the array

['1', '2', '3', '4', '5'] -> ['5', '4', '3', '2', '1']

4

Join array back to string

['5', '4', '3', '2', '1'] -> '54321'

5

Convert string to number

Number('54321') -> 54321

Step	Value
String(num)	'12345'
Split	['1','2','3','4','5']
Reverse	['5','4','3','2','1']
Join	'54321'
Number	54321

Why This Works

Step 1: Convert number to string

Using String(num) lets us treat the number like text so we can work with each digit.

Step 2: Reverse the digits

Splitting into an array and reversing it flips the order of digits easily.

Step 3: Convert back to number

Joining the reversed array and converting back to a number gives the reversed numeric value.

Alternative Approaches

Using math and loops

javascript

function reverseNumber(num) {
  let reversed = 0;
  while (num > 0) {
    reversed = reversed * 10 + num % 10;
    num = Math.floor(num / 10);
  }
  return reversed;
}

console.log(reverseNumber(12345));

This method uses math operations without converting to string, which can be faster for large numbers but is a bit more complex.

Using recursion

javascript

function reverseNumber(num, rev = 0) {
  if (num === 0) return rev;
  return reverseNumber(Math.floor(num / 10), rev * 10 + num % 10);
}

console.log(reverseNumber(12345));

This recursive approach is elegant but can cause stack overflow for very large numbers.

Complexity: O(n) time, O(n) space

Time Complexity

The time depends on the number of digits n, as splitting, reversing, and joining each process all take O(n) time.

Space Complexity

Extra space is needed for the array of digits, so space complexity is O(n).

Which Approach is Fastest?

The math-based loop method uses O(1) space and can be faster for very large numbers, while string methods are simpler and more readable.

Approach	Time	Space	Best For
String methods	O(n)	O(n)	Simplicity and readability
Math and loops	O(n)	O(1)	Performance with large numbers
Recursion	O(n)	O(n)	Elegant code but limited by call stack