Reverse a number in JavaScript

Our aim is to write a JavaScript function that takes in a number and returns its reversed number.

For example, reverse of 678 is:

876

There are multiple approaches to reverse a number in JavaScript. Let's explore the most common methods.

Method 1: Using String Conversion

The most straightforward approach converts the number to a string, reverses it, and converts back to a number:

const num = 124323;
const reverse = (num) => parseInt(String(num)
    .split("")
    .reverse()
    .join(""), 10);
console.log(reverse(num));

323421

How It Works

Let's break down the string conversion method step by step:

• Let's say num = 123
• We convert the num to string ? num becomes '123'
• We split '123' ? it becomes ['1', '2', '3']
• We reverse the array ? it becomes ['3', '2', '1']
• We join the array to form a string ? it becomes '321'
• Finally we parse the string into an integer and return it ? 321

Method 2: Using Mathematical Approach

This method uses modulo and division operations without string conversion:

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));
console.log(reverseNumber(678));

54321
876

Method 3: Handling Negative Numbers

For a more robust solution that handles negative numbers:

function reverseWithSign(num) {
    const isNegative = num 

54321
-876
21


Comparison

Conclusion

The string conversion method is the most readable and commonly used approach for reversing numbers in JavaScript. For performance-critical applications, consider the mathematical approach.