freeCodeCamp/curriculum/challenges/english/99-dev-playground/daily-coding-challenges-javascript/javascript-challenge-12.md

id, title, challengeType, dashedName

id	title	challengeType	dashedName
68216ef80f957572e7c340c5	JavaScript Challenge 12: Message Decoder	28	javascript-challenge-12

--description--

Given a secret message string, and an integer representing the number of letters that were used to shift the message to encode it, return the decoded string.

• A positive number means the message was shifted forward in the alphabet.
• A negative number means the message was shifted backward in the alphabet.
• Case matters, decoded characters should retain the case of their encoded counterparts.
• Non-alphabetical characters should not get decoded.

--hints--

decode("Xlmw mw e wigvix qiwweki.", 4) should return "This is a secret message."

assert.equal(decode("Xlmw mw e wigvix qiwweki.", 4), "This is a secret message.");

decode("Byffi Qilfx!", 20) should return "Hello World!"

assert.equal(decode("Byffi Qilfx!", 20), "Hello World!");

decode("Zqd xnt njzx?", -1) should return "Are you okay?"

assert.equal(decode("Zqd xnt njzx?", -1), "Are you okay?");

decode("oannLxmnLjvy", 9) should return "freeCodeCamp"

assert.equal(decode("oannLxmnLjvy", 9), "freeCodeCamp");

--seed--

--seed-contents--

function decode(message, shift) {

  return message;
}

--solutions--

function decode(message, shift) {
  return message.split('').map(char => {
    if (/[a-zA-Z]/.test(char)) {
      const base = char === char.toLowerCase() ? 'a'.charCodeAt(0) : 'A'.charCodeAt(0);
      const charCode = char.charCodeAt(0);
      const offset = (charCode - base - shift + 26) % 26;
      return String.fromCharCode(base + offset);
    } else {
      return char;
    }
  }).join('');
}