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

We are given a string, our task is to find string is palindrome or not. A palindrome is a series of numbers, strings, or letters that, when read from right to left and left to right, match each other exactly or produce the same series of characters. in simple words when number strings or characters are inverted and still provide the same outcome as the original numbers or characters. 

Example:

Input: "race"
Output: passed string is not a palindrome
Explanation: if we write "race" in reverse that is "ecar" it not 
matches with first string so it is not a palindrome.
Input: "hellolleh"
Output: passed string is palindrome.

Below are the methods by which we can check whether a passed string is palindrome or not:

By using a Naive Approach

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

Example: This example shows the use of the above-explained approach.

// Function that check str is palindrome 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 palindrome
    return true;

}

// Function that print output if string is palindrome
function is_palindrome(str) {
    
    // Variable that is true if string is palindrome
    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_palindrome(test);

passed string is palindrome 

By reversing the string

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

Follow the following steps : 

• Initialize reverse_str a variable that stores the reverse of the passed string.
• Compare the string to reverse_str.
• If matches, it is a palindrome.
• The else string is not a palindrome.

Example: This example shows the use of the above-explained approach.

// 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);

passed string is palindrome 

Using the split(), reverse(), and join() Methods

Another approach, which is through the shortest approach, uses the split(), reverse(), and join() methods.

• Split the string of characters into several different characters (which is though unsorted at the moment).
• Use the reverse() method to reverse all the characters of the string alphabetically.
• Then apply the join() method in order to join all the characters of the string (which are now sorted).

Example: Below is the implementation of the above approach:

let checkPalindrome = (stringg) => {
    return stringg === stringg.split("").reverse().join("");
};

console.log("Is Palindrome? : " + checkPalindrome("noon"));
console.log("Is Palindrome?: " + checkPalindrome("apple"));

Is Palindrome? : true
Is Palindrome?: false

Using Array.every()

Using Array.every(), the approach iterates over the characters of the string from both ends simultaneously, comparing each pair. If all pairs match, it returns true, indicating the string is a palindrome; otherwise, false.

Example: Here The function isPalindrome() checks if every character matches its corresponding character from the end. Returns true if palindrome, false otherwise.

function isPalindrome(str) {
  return str.split('').every((char, index) => char === str[str.length - index - 1]);
}


console.log(isPalindrome("radar")); 
console.log(isPalindrome("hello")); 
console.log(isPalindrome("level")); 

true
false
true

Using Two Pointers

This approach uses two pointers, one starting from the beginning of the string and the other from the end, moving towards the center, comparing characters along the way.

Example:

function isPalindrome(str) {
    let left = 0;
    let right = str.length - 1;
    
    while (left < right) {
        if (str[left] !== str[right]) {
            return false;
        }
        left++;
        right--;
    }
    return true;
}

console.log(isPalindrome("madam")); // Output: true
console.log(isPalindrome("test"));  // Output: false
console.log(isPalindrome("abba"));  // Output: true

true
false
true