deno fmt --check: check formatting in CI without changing files

deno fmt --check checks whether files are formatted, prints a diff for each one that is not, and exits with code 1 without changing anything. Plain deno fmt rewrites the files in place. Put deno fmt --check in CI next to deno lint so unformatted code fails the build; run deno fmt locally to fix it.

Check, fix, check again

printf 'const x = {a:1,\n  b:2}\nconsole.log( x )\n' > main.ts
npx --yes deno@2 fmt --check main.ts > check.log 2>&1
echo "check exit=$?"
tail -n 1 check.log
npx --yes deno@2 fmt main.ts 2>&1
cat main.ts
npx --yes deno@2 fmt --check main.ts 2>&1
echo "check exit=$?"

check exit=1
error: Found 1 not formatted file in 1 file
/tmp/work/main.ts
Checked 1 file
const x = { a: 1, b: 2 };
console.log(x);
Checked 1 file
check exit=0

Without file arguments, deno fmt walks the current directory, skipping files ignored by .gitignore and the fmt.exclude list in deno.json.

Useful options

The same settings can live in deno.json so every developer and CI job uses them:

{
  "fmt": {
    "lineWidth": 100,
    "singleQuote": true,
    "exclude": ["dist/"]
  }
}

Svelte, Vue, Astro and Angular templates need --unstable-component, and SQL files --unstable-sql. Deno also reads .editorconfig for indentation settings unless you pass --no-editorconfig.

Supported files

deno fmt formats much more than TypeScript, which is why --check can fail on a file you did not expect:

To keep a generated or vendored folder out of the check, list it in fmt.exclude in deno.json or pass --ignore.

In a CI workflow

Run the check before the slower steps, so a formatting mistake fails the job in seconds:

- run: deno fmt --check
- run: deno lint
- run: deno test

--check reports every unformatted file with a diff; add --fail-fast in large repositories to stop at the first one. A failing check is fixed by running deno fmt locally and committing the result, so pin the same Deno version locally and in CI: formatter output can change between releases.

FAQ