JavaScript Program to Check Palindrome String or Number

A palindrome is a string or number that reads the same forwards and backwards — "madam", "racecar", and 121 are all palindromes. Checking for one is a direct application of reversing a string: reverse the input and compare it to the original.

Check Palindrome String in JavaScript

function isPalindrome(str) {
  // Normalize: lowercase and strip anything that isn't a letter or digit
  const cleaned = str.toLowerCase().replace(/[^a-z0-9]/g, "");
  const reversed = cleaned.split("").reverse().join("");
  return cleaned === reversed;
}

console.log(isPalindrome("Madam"));
console.log(isPalindrome("Hello"));
console.log(isPalindrome("A man a plan a canal Panama"));

Output:

true
false
true

Normalizing the string first — lowercasing it and removing spaces/punctuation with a regular expression — is what lets a phrase like "A man a plan a canal Panama" register as a palindrome. Skip that step and it would fail, since the raw string has spaces and mixed case that don't mirror each other.

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Check Palindrome Number in JavaScript

Numbers don't have string methods directly, so the usual approach is to convert the number to a string first, reuse the same reverse-and-compare logic, then convert back if you need a numeric result.

function isPalindromeNumber(num) {
  const str = num.toString();
  const reversed = str.split("").reverse().join("");
  return str === reversed;
}

console.log(isPalindromeNumber(121));
console.log(isPalindromeNumber(123));

Output:

true
false

Negative numbers are a common edge case worth testing: (-121).toString() is "-121", whose reverse is "121-" — so this function correctly reports negative numbers as non-palindromes, since the minus sign breaks the symmetry.

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