isPalindromeMyVersion.js

/*-----------------------------------------------------------------------------
1.check if the string that reversed is as same as the origin.
2.ignoring the situation like upper or lower, and punctuation.

the reference used at the second function is from UdemyCourse: LearningAlgorithmsInJavascriptFromScratch
-----------------------------------------------------------------------------*/

function isPalindrome(string) {
  var reg = /[a-z]/g; 
  
  // change all to lower, focus on the alphabet, and put them together.
  var normalStr = string.toLowerCase().match(reg).join("");
  
  // one more step than the normalStr: reverse()  
  var reversedStr = string.toLowerCase().match(reg).reverse().join("");
  
  return normalStr === reversedStr;
}
isPalindrome("aabbddsddbbaa");

isPalindromeOfficialVersion.js

function isPalindrome2(string) {
  string = string.toLowerCase();
  var characterArr = string.split("");
  
  var validCharacters = "abcdefghijklmnopqrstuvwxyz".split("");
  
  var letterArr = [];
  characterArr.forEach((char) => {
    if (validCharacters.indexOf(char) > -1) {
      letterArr.push(char);
    }
  });
   if (letterArr.join("") === letterArr.reverse().join("")) return true;
    else return false;
}
isPalindrome2("aabbddsddbbaa");