Check for Palindromes -- why does this code work, but not the other?


I’m working through the palindromes algorithm on Front End, and came up with the following:

function palindrome(str) {
  if (str == str.replace(/[\W_]/g, "").toLowerCase().split("").reverse().join("")) {
    return true;

  else {
    return false;


It doesn’t work for many cases, particularly where there’s white space and underscores.

Yet this code does:

function palindrome(str) {
  return str.replace(/[\W_]/g, "").toLowerCase() ===
         str.replace(/[\W_]/g, "").toLowerCase().split("").reverse().join("");


What is the “===” doing here that is making it pass? Why can’t the first code pass as written?


Hi @asherem

If you compare the 2 pieces of code, they’re actually a fair bit different than just using ‘==’ and ‘===’.
FYI the triple equals checks value AND type, so:

5 == '5' // true
5 === '5' // false

Anyway, the if you compact the first piece of code to be similar to the second, you’ll see what I mean.

// First piece of code
return str == 
       str.replace(/[\W_]/g, "").toLowerCase().split("").reverse().join("")

// Second piece of code
return str.replace(/[\W_]/g, "").toLowerCase() ===
       str.replace(/[\W_]/g, "").toLowerCase().split("").reverse().join("");

The first piece of code isn’t removing the non-alphanumeric characters, nor making them all lower case. That’s why it’s not behaving the same. In either instance, using == / === isn’t making a difference, although it’s always best practice to use ===.

I still don’t understand. Unless I’m missing the logic, the implicit meaning in “str == str.replace…” is “if the original str is equivalent to str.replace…, return true” - I don’t see why actually making those changes to the original string is necessary?

I’m coming from Python, which behaves differently, so maybe that’s why. This is a similar function (minus for the reg ex) in Python, which works fine:

def palindrome(string):
    if string == string[::-1].lower():
        return True
        return False

So, is this merely a difference between Python and JS that I’m not seeing?

From challenge description:

A palindrome is a word or sentence that’s spelled the same way both forward and backward, ignoring punctuation, case, and spacing.
You’ll need to remove all non-alphanumeric characters (punctuation, spaces and symbols) and turn everything lower case in order to check for palindromes.

Let’s take 'eye_'
When you do str == str.replace... you are comparing eye_ with eye.

1 Like

It’s not really a difference in the languages, I think you’re misunderstanding the challenge itself. As @jenovs said, you need to remove all the non-alpha characters BEFORE you check to see if it’s a palindrome. So what you’re really meant to do is check a modified version (non-alphas removed and lower cased) of the original string with it reversed, The 2nd piece of code does just that.

I see - woke up today, and saw the issue right away. Thanks.

I have an issue with my code, maybe someone can help. When I return reverseStr I get as result :eye plus a “,” , I have no idea where is this coming from

function palindrome(str) {
var newPalindrome = str.replace(/[^A-Za-z0-9]/g,’’);
var lowCaseString = newPalindrome.toLowerCase();
var reverseStr =lowCaseString.split(’’);
var finalString= reverseStr.reverse().join(’’);
if(finalString == reverseStr) return true;
else if (finalString != reverseStr)return false;

Thanks for tue input!

If you put the following console.log statements on a new line above the if statement:

  console.log(reverseStr); // yields [ 'eye', '' ]
  console.log(finalString); // yields 'eye '

you would see find that reverseStr is an array and finalString is a string. You need to be comparing two strings.