Where is my implementation of rot13 in JavaScript going wrong?

Solution 1:

You could use the super-short:

s.replace(/[a-zA-Z]/g,function(c){return String.fromCharCode((c<="Z"?90:122)>=(c=c.charCodeAt(0)+13)?c:c-26);});

Solution 2:

Here's a solution using replace and indexOf functions:

function rot13(s) {
  return s.replace(/[A-Z]/gi, c =>
    "NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm"[
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz".indexOf(c) ] )
}

This is made up of:

• /[A-Z]/gi regular expression for matching only characters
• replace is used to substitute those characters
• an a replacer function written as an arrow function
• indexOf is to convert the character into a numeric lookup index
• we lookup the index in the substitution array and we're done

Solution 3:

This gives correct results.

function rot13(s)
 {
    return (s ? s : this).split('').map(function(_)
     {
        if (!_.match(/[A-Za-z]/)) return _;
        c = Math.floor(_.charCodeAt(0) / 97);
        k = (_.toLowerCase().charCodeAt(0) - 83) % 26 || 26;
        return String.fromCharCode(k + ((c == 0) ? 64 : 96));
     }).join('');
 }
 
 alert(rot13(rot13("Mark this as accepted answer :)")));

Solution 4:

Just because it's even shorter and also more understandable/logical:

function rot13(s) {
  return s.replace( /[A-Za-z]/g , function(c) {
    return String.fromCharCode( c.charCodeAt(0) + ( c.toUpperCase() <= "M" ? 13 : -13 ) );
  } );
}