Caesar shift in JavaScript

gistfile1.js

/*

JavaScript Caesar shift
by Evan Hahn (evanhahn.com)

"Encrypt" like this:

    caesarShift('Attack at dawn!', 12);    // Returns "Mffmow mf pmiz!"

And "decrypt" like this:

    caesarShift('Mffmow mf pmiz!', -12);    // Returns "Attack at dawn!"

For simplicity, only works with ASCII characters.

* * * * * * * * * * * *

This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or distribute
this software, either in source code form or as a compiled binary, for any
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*/

var caesarShift = function (str, amount) {
  // Wrap the amount
  if (amount < 0) {
    return caesarShift(str, amount + 26);
  }

  // Make an output variable
  var output = "";

  // Go through each character
  for (var i = 0; i < str.length; i++) {
    // Get the character we'll be appending
    var c = str[i];

    // If it's a letter...
    if (c.match(/[a-z]/i)) {
      // Get its code
      var code = str.charCodeAt(i);

      // Uppercase letters
      if (code >= 65 && code <= 90) {
        c = String.fromCharCode(((code - 65 + amount) % 26) + 65);
      }

      // Lowercase letters
      else if (code >= 97 && code <= 122) {
        c = String.fromCharCode(((code - 97 + amount) % 26) + 97);
      }
    }

    // Append
    output += c;
  }

  // All done!
  return output;
};

Or in ES:

const caesar = (text, shift) => {
  return String.fromCharCode(
    ...text.split('').map(char => ((char.charCodeAt() - 97 + shift) % 26) + 97),
  );
};

I did not understood this expression
"((code - 65 + amount) % 26) + 65)" on line no 76 can you help me out.
thanks for giving me time .

There is only 26 characters in ascii, if you don't mod encrypt string by 26, encyrpted string will fall outside ascii range. for example ç => % or z = İ etc.
If you mod it by 26, encrypted string will be only ascii too, so it is guarentied it'll be human-readable. If you don't care about human readability of encrypted string, you may take it as optional.

const caesar = (text, shift) => {
  return String.fromCharCode(
    ...text.split('').map(char => (char.charCodeAt() + shift)),
  );
};

Btw, 65 and 95 is for ascii range of lowercase and uppercase characters.

@alimertcakar Good point. I've added a comment mentioning that this is ASCII-only.

I understand all of this, but why should i divide by 26? I considered manually, some garbage comes out.

I understand all of this, but why should i divide by 26? I considered manually, some garbage comes out.

I also don't understand this part

I wrote this a long time ago and don't have time to help debug this, sadly.

const caesar = (text, shift) => {
  return String.fromCharCode(
    ...text.split('').map(char => ((char.charCodeAt() - 65 + shift) % 26) + 65),
  );
};

caesar("ABCDEFGHIJKLMNOPQRSTUVWXYZ", 13) // NOPQRSTUVWXYZABCDEFGHIJKLM

2020 Typescript Version:

// Caesar Cipher
const caesarCipher = (string: string, shift: number) => {
  // Alphabet
  const alphabet: string = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';

  // Encoded Text
  let encodedText: string = '';

  // Adjust Shift (Over 26 Characters)
  if (shift > 26) {
    // Assign Remainder As Shift
    shift = shift % 26;
  }

  // Iterate Over Data
  let i: number = 0;
  while (i < string.length) {
    // Valid Alphabet Characters
    if (alphabet.indexOf(string[i]) !== -1) {
      // Find Alphabet Index
      const alphabetIndex: number = alphabet.indexOf((string[i]).toUpperCase());

      // Alphabet Index Is In Alphabet Range
      if (alphabet[alphabetIndex + shift]) {
        // Append To String
        encodedText += alphabet[alphabetIndex + shift];
      }
      // Alphabet Index Out Of Range (Adjust Alphabet By 26 Characters)
      else {
        // Append To String
        encodedText += alphabet[alphabetIndex + shift - 26];
      }
    }
    // Special Characters
    else {
      // Append To String
      encodedText += string[i];

    }

    // Increase I
    i++;
  }

  return encodedText;
};

Example #1:

console.log(caesarCipher('ABCDEFGHIJKLMNOPQRSTUVWXYZ', 2));

CDEFGHIJKLMNOPQRSTUVWXYZAB

Example #2:

console.log(caesarCipher('GUR DHVPX OEBJA QBT WHZCRQ BIRE GUR YNML SBK.', 26 + 13));

THE QUICK BROWN DOG JUMPED OVER THE LAZY FOX.

Thank you! I've been struggling a lot with this exercise. Your code is wonderfully clean and instructive, I managed to get mine working thanks to you!

1 line Caesar Cypher in ES6 using inbuilt Array.prototype.replace()
Console.log at every turn to understand the inner mechanism. Happy coding!

@mosemet, your solution works only with min characters, a more complete solution :

function shift(letter, k, code) {
    return String.fromCharCode(((letter.charCodeAt() - code + k) % 26) + code);
}

function caesarCipher(s, k) {
    return s.replace(/[a-z]/g, char => shift(char, k, 97)).replace(/[A-Z]/g, char => shift(char, k, 65));
}

-------------------------- OR --------------------------

function isCharacterAMinLetter(char) {
  return (/[a-z]/).test(char)
}

function isCharacterAMajLetter(char) {
  return (/[A-Z]/).test(char)
}

function shift(letter, k, code) {
   return String.fromCharCode(((letter.charCodeAt() - code + k) % 26) + code);
}

return s.replace(/[A-Za-z]/g, char => {
  // check if min letter
  if (isCharacterAMinLetter(char)) {
      return shift(char, k, 97);
  } 
  // check if maj letter
  else if (isCharacterAMajLetter(char)) {
      return shift(char, k, 65);
  }
});

It's working perfectly. Thanks Evan for the awesome solution!!!

function rotateAlphabet(text, noOfChars = 0){
    const n = noOfChars % text.length;
    return text.slice(n) + text.slice(0, n); 
}

function convertText(string, noOfChars = 0) {
    const alphabetLowercase         = 'abcdefghijklmnopqrstuvwxyz'
    const alphabetRotatedLowercase  = rotateAlphabet(alphabetLowercase, noOfChars)
    const alphabetUppercase         = alphabetLowercase.toUpperCase()
    const alphabetRotatedUppercase  = rotateAlphabet(alphabetUppercase, noOfChars)
    let arrayResult                 = []
    let index                       = 0
    
    for (let i = 0; i < string.length; i++) {
        // if character is NOT alphanumeric
        if (! /^[a-zA-Z]+$/.test(string[i])) {  
            arrayResult.push(string[i]) 
        } else if (string[i] == string[i].toLowerCase()) {
            // if character is lowercase
            index = (alphabetLowercase.indexOf(string[i]))
            arrayResult.push((alphabetRotatedLowercase[index]))  
        } else if (string[i] == string[i].toUpperCase()) {
            // else if character is uppercase
            index = (alphabetUppercase.indexOf(string[i]))
            arrayResult.push((alphabetRotatedUppercase[index])) 
        }
    }
    return arrayResult.join('')
}

@mosemet, your solution works only with min characters, a more complete solution :

function shift(letter, k, code) {
    return String.fromCharCode(((letter.charCodeAt() - code + k) % 26) + code);
}

function caesarCipher(s, k) {
    return s.replace(/[a-z]/g, char => shift(char, k, 97)).replace(/[A-Z]/g, char => shift(char, k, 65));
}

-------------------------- OR --------------------------

function isCharacterAMinLetter(char) {
  return (/[a-z]/).test(char)
}

function isCharacterAMajLetter(char) {
  return (/[A-Z]/).test(char)
}

function shift(letter, k, code) {
   return String.fromCharCode(((letter.charCodeAt() - code + k) % 26) + code);
}

return s.replace(/[A-Za-z]/g, char => {
  // check if min letter
  if (isCharacterAMinLetter(char)) {
      return shift(char, k, 97);
  } 
  // check if maj letter
  else if (isCharacterAMajLetter(char)) {
      return shift(char, k, 65);
  }
});

not safe to use, z will be decoded to ` instead of a