Palindromes

Palindromes:

A palindrome is a word, phrase, number, or other sequence of characters which reads the same backward or forward. 

I'll show two approaches:

  1st  with built-in functions,
  2nd  using a for loop.

 Solution 1:
Palindromes With Built-In Functions.

 function palindrome(s__1) {
     // Step 1. Lowercase the string and use the RegExp to remove the unwanted characters from it
     var re = /[\W_]/g; // or var re = /^[A-Za-z0-9]/g;
    
     var lowRegStr = s__1.toLowerCase().replace(re, '');
     // s__1.toLowerCase() = "race car".toLowerCase() = "race car"
     // s__1.replace(/[\W_]/g, '') = "race car".replace(/[\W_]/g, '') = "racecar"
     // var lowRegStr = "racecar"
    
    
     // Step 2. Use the methods with buid-in functions
     // to reverse a string
     var reverseStr = lowRegStr.split('').reverse().join('');
    
    
     // Strp 3. Check if reverseStr is strictly equals to lowRegStr and return a Boolean
     return reverseStr === lowRegStr; // "racecar" === "racecar"? => true
    
 }

 Solution 2:
Check for Palindromes With a FOR loop.

Half-indexing (len/2) has benefits when processing large strings. We check the end from each part and divide the number of iterations inside the FOR loop by two. 

 function palindrome2(s__2) {
     // Step 1. The first part is the same as previous solution(1)
     var reg = /[^A-Za-z0-9]/g; // or var reg = /[\W_]/g;

     s__2 = s__2.toLowerCase().replace(reg, ''); // 'madam'


     // Step 2. Create the FOR loop
     var len = s__2.length; // var len = "Madam".length = 5  ("Race car".length = 8)

     for (var i = 0; i < len/2; i++) {
         // As long as the characters from each part match, the FOR loop will go on
         if (s__2[i] !== s__2[len -1 -i]) {
             // When the characters don't match anymore, false is returned and we exit the FOR loop
             return false;
         }
     }
    
     return true; // Both parts are strictly equal, it returns true => The string is palindrome

 }