Workshop › Text detective › Step 10 of 13

Shift a letter

What we are doing

Now a secret code: the Caesar cipher replaces each letter with the one 3 places later in the alphabet. First, a function that shifts one letter.

```js
const alphabet = 'abcdefghijklmnopqrstuvwxyz'

function shiftLetter(letter, shift) {
  const i = alphabet.indexOf(letter)
  if (i === -1) return letter
  return alphabet[(i + shift) % 26]
}
```

What it means

• alphabet.indexOf(letter) is the letter's position (a is 0, z is 25), or -1 if it is not a letter.
• Anything that is not a letter (space, dot) comes back as it is.
• % 26 is the remainder after dividing by 26: 25 + 3 = 28 becomes 2, so z turns into c.

Your turn

Write the alphabet and shiftLetter at the top of the script, before box.

Skills: Functions

← The most common word Encode a whole text →

All steps

1. Read the text
2. Count the characters
3. Update as you type
4. Split into words
5. The extra-space trap
6. An empty box has no words
7. Count each word
8. Drop the punctuation
9. The most common word
10. Shift a letter
11. Encode a whole text
12. The secret line
13. Build it yourself: crack the code