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Caesar shift in JavaScript
/*
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
purpose, commercial or non-commercial, and by any means.
In jurisdictions that recognize copyright laws, the author or authors of this
software dedicate any and all copyright interest in the software to the public
domain. We make this dedication for the benefit of the public at large and to
the detriment of our heirs and successors. We intend this dedication to be an
overt act of relinquishment in perpetuity of all present and future rights to
this software under copyright law.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
For more information, please refer to <https://unlicense.org/>
*/
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;
};
@Erbolking
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Erbolking commented Jan 25, 2018

You make my day! Thanks a lot!

@sanketsingh27
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sanketsingh27 commented Mar 11, 2018

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 .

@embed0
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embed0 commented Mar 25, 2018

You want to move it to the left by 65 so A is no code 65 but 0 so you have
0 A
1 B
2 C ...
so then you can easily go to the begining of alphabet when you go past 25th letter which has code 90 - Z (when moved to the left by 65 it's code is 25)
But then at the end you have to add again this 65 so you have your letter code and not the position in alphabet.

@DaNgLiN
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DaNgLiN commented Feb 28, 2019

Thanks man You save me

@sethidden
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sethidden commented Mar 10, 2019

Or in ES:

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

@alimertcakar
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alimertcakar commented Jun 12, 2020

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.

@EvanHahn
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Author

EvanHahn commented Jun 14, 2020

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

@Hmekus
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Hmekus commented Jul 12, 2020

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

@Corey-Wademan
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Corey-Wademan commented Oct 7, 2020

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

@EvanHahn
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Author

EvanHahn commented Oct 8, 2020

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

@jjarcik
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jjarcik commented Nov 19, 2020

const caesar = (text, shift) => {
  return String.fromCharCode(
    ...text.split('').map(char => ((char.charCodeAt() - 65 + shift) % 26) + 65),
  );
};
caesar("ABCDEFGHIJKLMNOPQRSTUVWXYZ", 13) // NOPQRSTUVWXYZABCDEFGHIJKLM

@jefelewis
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jefelewis commented Nov 26, 2020

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.

@muratcan-yuksel
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muratcan-yuksel commented Dec 22, 2020

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!

@mosemet
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mosemet commented Aug 19, 2021

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

1lineceasar

@DalyIng
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DalyIng commented Sep 21, 2021

@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);
  }
});

@ashwaniw
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ashwaniw commented Dec 9, 2021

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

@JoshSalway
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JoshSalway commented May 2, 2022

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('')
}

@luatnd
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luatnd commented May 16, 2022

@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

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