1+ /**
2+ * Caesar's Cipher - also known as the ROT13 Cipher is when
3+ * a letter is replaced by the one that is 13 spaces away
4+ * from it in the alphabet. If the letter is in the first half
5+ * of the alphabet we add 13, if it's in the latter half we
6+ * subtract 13 from the character code value.
7+ */
8+ 9+ /**
10+ * Decrypt a ROT13 cipher
11+ * @param {String } str - string to be decrypted
12+ * @return {String } decrypted string
13+ */
14+ function rot13 ( str ) {
15+ let response = [ ] ;
16+ let strLength = str . length ;
17+ 18+ for ( let i = 0 ; i < strLength ; i ++ ) {
19+ const char = str . charCodeAt ( i ) ;
20+ 21+ switch ( true ) {
22+ // Check for non-letter characters
23+ case char < 65 || ( char > 90 && char < 97 ) || char > 122 :
24+ response . push ( str . charAt ( i ) ) ;
25+ break ;
26+ // Letters from the second half of the alphabet
27+ case ( char > 77 && char <= 90 ) || ( char > 109 && char <= 122 ) :
28+ response . push ( String . fromCharCode ( str . charCodeAt ( i ) - 13 ) ) ;
29+ break ;
30+ // Letters from the first half of the alphabet
31+ default :
32+ response . push ( String . fromCharCode ( str . charCodeAt ( i ) + 13 ) ) ;
33+ }
34+ }
35+ return response . join ( '' ) ;
36+ }
37+ 38+ 39+ // Caesars Cipher Example
40+ const encryptedString = 'Uryyb Jbeyq' ;
41+ const decryptedString = rot13 ( encryptedString ) ;
42+ 43+ console . log ( decryptedString ) ; // Hello World
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