Nearest palindrome in JavaScript

We are required to write a function that takes in a number n and returns a palindromic number that is nearest to the number n. A palindrome reads the same forwards and backwards.

For example:

• If the input number is 264, then the output should be 262

• If the input number is 7834, then the output should be 7887

Approach

The approach divides the number into two halves based on its length and creates a palindrome by mirroring the first half. For odd-length numbers, the middle digit remains unchanged.

Example

const findNearestPalindrome = num => {
    const strNum = String(num);
    const half = strNum.substring(0, Math.floor(strNum.length/2));
    const reversed = half.split("").reverse().join("");
    const first = strNum.length % 2 === 0 ? half : strNum.substring(0,
    Math.ceil(strNum.length/2))
    return +(first+reversed);
};

console.log(findNearestPalindrome(235));
console.log(findNearestPalindrome(23534));
console.log(findNearestPalindrome(121));
console.log(findNearestPalindrome(1221));
console.log(findNearestPalindrome(45));

Output

232
23532
121
1221
44

How It Works

Let's break down the algorithm with an example:

const findNearestPalindrome = num => {
    const strNum = String(num);
    console.log(`Input: ${num}, String: "${strNum}"`);
    
    const half = strNum.substring(0, Math.floor(strNum.length/2));
    console.log(`First half: "${half}"`);
    
    const reversed = half.split("").reverse().join("");
    console.log(`Reversed half: "${reversed}"`);
    
    const first = strNum.length % 2 === 0 ? half : strNum.substring(0,
    Math.ceil(strNum.length/2));
    console.log(`Complete first part: "${first}"`);
    
    const result = +(first+reversed);
    console.log(`Final palindrome: ${result}<br>`);
    return result;
};

findNearestPalindrome(235);
findNearestPalindrome(1234);

Input: 235, String: "235"
First half: "2"
Reversed half: "2"
Complete first part: "23"
Final palindrome: 232

Input: 1234, String: "1234"
First half: "12"
Reversed half: "21"
Complete first part: "12"
Final palindrome: 1221

Key Points

• For even-length numbers: Split in half and mirror the first half

• For odd-length numbers: Keep the middle digit and mirror the first half

• The + operator converts the string result back to a number

Conclusion

This approach creates palindromes by mirroring the first half of a number. It works efficiently for most cases but may not always find the truly nearest palindrome for all inputs.