Write a Caesar Cipher in JavaScript (Exercise)

The challenge

Write caesar(text, shift) that shifts every lowercase letter forward in the alphabet by shift places, wrapping from z back to a. Leave everything else (spaces, punctuation) as it is.

Example

caesar("abc", 1)        // "bcd"
caesar("xyz", 3)        // "abc"
caesar("hi there!", 13) // "uv gurer!"

Try it

Write your answer below, then press Check my answer. Your code runs right here in your browser, and the tests tell you what works.

Your solution

It prints

What the tests check

caesar("abc", 1) gives "bcd"
caesar("xyz", 3) gives "abc"
caesar("hi there!", 13) gives "uv gurer!"
Shifting by 5 then 21 gets the original back: caesar(caesar("secret", 5), 21) gives "secret"

Hints

Stuck? Open one hint at a time. Try again after each one.

Hint 1

const ALPHABET = "abcdefghijklmnopqrstuvwxyz"; and ALPHABET.indexOf(char) gives a letter’s position.

Hint 2

Wrap around with the remainder: (index + shift) % 26.

A solution

There’s always more than one right way. Read it, make sure you understand it, then try writing it yourself without looking.

Show one possible solution

Solution

function caesar(text, shift) {
  const ALPHABET = "abcdefghijklmnopqrstuvwxyz";
  let result = "";
  for (const char of text) {
    const i = ALPHABET.indexOf(char);
    result += i === -1 ? char : ALPHABET[(i + shift) % 26];
  }
  return result;
}

Learn what you need

This exercise uses what Grade 10: Array Power Tools: map, filter & reduce teaches. Read those notes first if any of it is new.

Write a Caesar cipher