Checking an array for palindromes - JavaScript

We are required to write a JavaScript function that takes in an array of String / Number literals and returns a subarray of all the elements that were palindrome in the original array.

A palindrome is a word, number, or sequence that reads the same forward and backward. For example, "dad", "racecar", and 12321 are palindromes.

Problem Statement

If the input array is:

const arr = ['carecar', 1344, 12321, 'did', 'cannot'];

Then the output should be:

const output = [12321, 'did'];

Approach

We will create a helper function that takes in a number or a string and checks if it's a palindrome. Then we will loop over the array, filter the palindrome elements and return the filtered array.

Solution

const arr = ['carecar', 1344, 12321, 'did', 'cannot'];

const isPalindrome = el => {
    const str = String(el);
    let i = 0;
    let j = str.length - 1;
    
    while(i  {
    return arr.filter(el => isPalindrome(el));
};

console.log(findPalindrome(arr));

Output

[ 12321, 'did' ]

How It Works

The isPalindrome function converts any input to a string and uses two pointers (i and j) to compare characters from both ends moving toward the center. If all corresponding characters match, it returns true; otherwise false.

The findPalindrome function uses the filter method to create a new array containing only palindromic elements from the original array.

Alternative Using Reverse Method

const arr = ['carecar', 1344, 12321, 'did', 'cannot'];

const isPalindromeReverse = el => {
    const str = String(el);
    return str === str.split('').reverse().join('');
};

const findPalindromeReverse = arr => {
    return arr.filter(isPalindromeReverse);
};

console.log(findPalindromeReverse(arr));

Output

[ 12321, 'did' ]

Conclusion

Both approaches effectively identify palindromes in mixed arrays. The two-pointer method is more memory efficient, while the reverse method is more concise but creates additional string copies.