JavaScript Program to Reverse Digits of a Number

Reversing the digits of a number means rearranging its digits in the opposite order. For instance, reversing 1234 gives 4321. In JavaScript, this is typically achieved by converting the number to a string, reversing the characters, and converting it back to a number, offering efficient manipulation.

There are several methods that can be used to Reverse the Numbers by using JavaScript, which is listed below:

Using String Reversal

In this approach, we are using string reversal, converting a number to a string, reverse it using split('').reverse().join(''), and convert back to a number.

Syntax:

let result = num1.toString().split('').reverse().join('');

Example: In this example, we are using the above-explained approach.

let num1 = 123456789;
let result = num1.toString().split('').reverse().join('');
console.log(result);

987654321

Using Array Reduce() Method

In this approach, Using the reduce() method, reverse the digits of a number. Convert the number to an array of its digits, then employ the reduce() function with a spread operator to reverse the order.

Syntax

function reverseFunction(num) {
    let digits = Array.from(String(num), Number);
    let reversedArray = digits.reduce((acc, digit) =>
        [digit, ...acc], []);
    return parseInt(reversedArray.join(''));
}; 

Example: In this example the reverseFunction converts a number to an array of digits, reverses it, and joins it back into a number.

function reverseFunction(num) {
    let digits = Array.from(String(num), Number);
    let reversedArray = digits.reduce((acc, digit) =>
        [digit, ...acc], []);
    return parseInt(reversedArray.join(''));
}

let num = 123456789;
let reversedNum = reverseFunction(num);

console.log(reversedNum); 

987654321

Using String Iteration

In this approach, using string iteration,we convert number to string, iterate backward using a loop, and construct reversed string by appending digits. Convert back to a number.

Syntax

function reverseFunction(num) {
    let numStr = num.toString();
    let reversedStr = '';
    for (let i = numStr.length - 1; i >= 0; i--) {
        reversedStr += numStr[i];
    }
    return parseInt(reversedStr);
};

Example: In this example the reverseFunction converts a number to a string, reverses it character by character, and converts it back to a number.

function reverseFunction(num) {
    let numStr = num.toString();
    let reversedStr = '';
    for (let i = numStr.length - 1; i >= 0; i--) {
        reversedStr += numStr[i];
    }
    return parseInt(reversedStr);
}

let num = 987654321;
let reversedNum = reverseFunction(num);

console.log(reversedNum);

123456789

Using Recursion

In this approach,using recursion, reverse digits of a number. Recursively divide the number by 10, build reversed number by accumulating digits, and return the result.

Syntax

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

Example: In this example, the reverseFunction takes a number and a reversed parameter as inputs. It uses recursion to reverse the digits by repeatedly dividing the number by 10 to isolate the last digit, while building the reversed number by multiplying it by 10 and adding the current last digit.

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

const num = 987654321;
const result = reverseFunction(num);

console.log(result);

123456789