How to Reverse a Number in JavaScript

To reverse a number in JavaScript, you need to convert the number to a string, split it into an array, reverse the array and then join it back together. This guide will walk you through the process with code snippets for clarity.

Understanding the task

A number reversal flips the order of its digits. For example, reversing 123 yields 321. In JavaScript, since numbers can't be directly reversed, the operation is typically performed on strings.

Convert the number to a string

Start by converting the number to a string using the toString() method. This step is necessary because the reversal methods apply to strings and arrays, not numbers.

let number = 12345;
let stringNumber = number.toString();

Split the string into an array

Once the number is a string, use the split('') method to create an array where each element is a single digit.

let arrayDigits = stringNumber.split('');

Reverse the array

Use the reverse() method to invert the order of the elements in the array.

let reversedArrayDigits = arrayDigits.reverse();

Join the array back into a string

After reversing, use join('') to concatenate the elements back into a single string.

let reversedStringNumber = reversedArrayDigits.join('');

Convert the string back to a number

Lastly, convert the string back into a number with the parseInt() function. If you need to handle decimals, use parseFloat() instead.

let reversedNumber = parseInt(reversedStringNumber, 10);

Handling negative numbers

For negative numbers, you'll need to remove the minus sign before reversing and add it back after.

function reverseNumber(num) {
  let isNegative = num < 0;
  let reversedString = Math.abs(num).toString().split('').reverse().join('');
  let reversedNum = parseInt(reversedString, 10);
  return isNegative ? -reversedNum : reversedNum;
}

Dealing with decimals

If the number has decimals, the process is similar, but you'll need to handle the decimal point.

function reverseFloat(num) {
  let stringNumber = num.toString();
  let parts = stringNumber.split('.');
  let reversedInteger = parseInt(parts[0].split('').reverse().join(''), 10);
  if (parts.length === 2) {
    let reversedDecimal = parseInt(parts[1].split('').reverse().join(''), 10);
    return parseFloat(`${reversedInteger}.${reversedDecimal}`);
  }
  return reversedInteger;
}

Final thoughts

This guide provided a method to reverse both integers and floating-point numbers. It's important to note that JavaScript may not preserve leading zeros in a reversed number since they are not significant in a numerical representation.