How to check whether a passed string is palindrome or not in JavaScript ?

Last Updated : 09 Apr, 2021

Given a string, our task is to find string is palindrome or not.

Example:
Input : "race"
Output : passed string is not a palidrome
Explanation : if we write "race" in reverse that is "ecer" it not 
matches with first string so it is not a pallindrome.
Example 2:
Input : "hellolleh"
Output : passed string is palidrome.

Approach 1: In this approach we use following steps.

First we iterate over a string in forward and backward direction.
Check if all forward and backward character matches, return true.
If all forward and backward character does not matches, return false.
If return is true, it is a palindrome.

Example:

Javascript

<script>
  // function that check str is palindome or not 
  function check_palindrome( str )
  {
    let j = str.length -1;
    for( let i = 0 ; i < j/2 ;i++)
    {
      let x = str[i] ;//forward character 
      let y = str[j-i];//backward character
      if( x != y)
      {
        // return false if string not match
        return false;
      }
    }
    /// return true if string is palindome
    return true;
      
  }
  
  //function that print output is string is palindome
  function is_palidrome( str )
  {
    // variable that is true ig string is pallindrome
    let ans = check_palindrome(str);
    //condition checking ans is true or not 
    if( ans == true )
    {
      console.log("passed string is palindrome ");
    }
    else
    {
      console.log("passed string not a palindrome");
    }
  }
  // test variable
  let test = "racecar";
  is_palidrome(test);
</script>

Output :

passed string is palidrome.

Approach 2: Another approach is to reverse a string and check if the initial string matches with the reverse string or not.

Follow the following steps :

Initialize revese_str a variable which stores reverse of the passed string.
Compare the string to reverse_str .
If matches, it is a palindrome.
Else string is not a palindrome.

Example:

Javascript

<script>
  // function to reverse the string
  function reverse( str )
  {
    // variable holds reverse string 
    let rev_str = "";
    for( let i = str.length-1 ;i >= 0 ;i--)
    {
      rev_str+= str[i];
    }
    // return reverse string 
    return rev_str;
  }
  
  //  function checking string is palindrome or not 
  function is_palindrome( str )
  {
    reverse_str = reverse(str);
    //  condition checking if reverse str is 
    // same as string it is palindrome
    // else not a palindrome
    if( reverse_str === str)
    {
      console.log("passed string is palindrome ");
    }
    else
    {
      console.log("passed string is not palindrome")
    }
  }
  let test = "hellolleh";
  is_palindrome(test);
</script>

Output :

passed string is palidrome.

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