JavaScript(JS) JS check whether a string is palindrome or not

To check if a string is a palindrome, you need to check if it reads the same backward as forward. Here's one way to do it in JavaScript:

function isPalindrome(str) {
  // convert string to lowercase and remove non-alphanumeric characters
  str = str.toLowerCase().replace(/[\W_]/g, '');
  
  // reverse the string
  var reversedStr = str.split('').reverse().join('');
  
  // check if the original string and the reversed string are the same
  return str === reversedStr;
}

// test the function
console.log(isPalindrome("A man, a plan, a canal, Panama!")); // true
console.log(isPalindrome("Hello, world!")); // false

In this implementation, the isPalindrome() function takes a string as input, converts it to lowercase, and removes all non-alphanumeric characters using a regular expression. Then, it splits the string into an array of characters, reverses the order of the characters, and joins them back into a string. Finally, it compares the original string and the reversed string, and returns true if they are the same, and false otherwise.