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Caesar Cipher CLI

A Node CLI that shifts letters by a configurable key for encode or decode, preserving case and non-letters, with round-trip tests.

node --version

node caesar.mjs encode 3 "Attack at dawn!"

node --test caesar.test.mjs

Prints the shifted text, for example Dwwdfn dw gdzq! for a shift of 3 applied to Attack at dawn!.

What it does

Rotates letters within A-Z and a-z by the given shift and leaves every other character as-is. Decoding applies the inverse shift, so encode followed by decode returns the original text.

const ALPHA = 'abcdefghijklmnopqrstuvwxyz';

export function shift(text, key) {
  const k = ((key % 26) + 26) % 26;
  return [...text].map((ch) => {
    const lower = ch.toLowerCase();
    const idx = ALPHA.indexOf(lower);
    if (idx === -1) return ch;
    const out = ALPHA[(idx + k) % 26];
    return ch === lower ? out : out.toUpperCase();
  }).join('');
}

const [mode, keyArg, ...rest] = process.argv.slice(2);
const key = Number(keyArg);
if (!Number.isInteger(key)) { console.error('usage: caesar <encode|decode> <key> <text>'); process.exit(1); }
const text = rest.join(' ');
console.log(shift(text, mode === 'decode' ? -key : key));

Run and verify

node caesar.mjs encode 3 "Attack at dawn!"

Run the test command to check the implementation: node --test caesar.test.mjs.

Expected Output

Each letter moves three positions forward; case is preserved and punctuation is untouched:

Dwwdfn dw gdzq!

Limitations

The Caesar cipher provides no security; it is a teaching tool and is trivially broken by brute force over the 26 possible keys.