Palindrome check in Javascript

Palindrome check in Javascript

Maybe I will suggest alternative solution:

function checkPalindrom (str) {
  return str == str.split().reverse().join();

UPD. Keep in mind however that this is pretty much cheating approach, a demonstration of smart usage of language features, but not the most practical algorithm (time O(n), space O(n)). For real life application or coding interview you should definitely use loop solution. The one posted by Jason Sebring in this thread is both simple and efficient (time O(n), space O(1)).

25x faster than the standard answer

function isPalindrome(s,i) {
 return (i=i||0)<0||i>=s.length>>1||s[i]==s[s.length-1-i]&&isPalindrome(s,++i);

use like:


as it defines i itself


This is ~25 times faster than the standard answer below.

function checkPalindrome(str) {
  return str == str.split().reverse().join();


View console for performance results.

Although the solution is difficult to read and maintain, I would recommend understanding it to demonstrate non-branching with recursion and bit shifting to impress your next interviewer.


The || and && are used for control flow like if else. If something left of || is true, it just exits with true. If something is false left of || it must continue. If something left of && is false, it exits as false, if something left of a && is true, it must continue. This is considered non-branching as it does not need if-else interupts, rather its just evaluated.

1. Used an initializer not requiring i to be defined as an argument. Assigns i to itself if defined, otherwise initialize to 0. Always is false so next OR condition is always evaluated.

(i = i || 0) < 0

2. Checks if i went half way but skips checking middle odd char. Bit shifted here is like division by 2 but to lowest even neighbor division by 2 result. If true then assumes palindrome since its already done. If false evaluates next OR condition.

i >= s.length >> 1

3. Compares from beginning char and end char according to i eventually to meet as neighbors or neighbor to middle char. If false exits and assumes NOT palindrome. If true continues on to next AND condition.

s[i] == s[s.length-1-i]

4. Calls itself again for recursion passing the original string as s. Since i is defined for sure at this point, it is pre-incremented to continue checking the strings position. Returns boolean value indicating if palindrome.



A simple for loop is still about twice as fast as my fancy answer (aka KISS principle)

function fastestIsPalindrome(str) {
  var len = Math.floor(str.length / 2);
  for (var i = 0; i < len; i++)
    if (str[i] !== str[str.length - i - 1])
      return false;
  return true;

Palindrome check in Javascript

First problem

= is assign
== is compare

Second problem, Your logic here is wrong


You are subtracting one from the charAt and not the length.

Third problem, it still will be wrong since you are not reducing the length by i.

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