deno run --check: type-check TypeScript before running it

deno run main.ts does not type-check: since Deno 2.0 it strips the TypeScript types and runs the code, even if the types are wrong. deno run --check main.ts runs the TypeScript compiler first and refuses to start when there is a type error, printing TS error codes with the file and line. deno check main.ts does the same check without running anything, which is what CI usually wants.

Without and with --check

count is declared as a number but receives a string:

cat > main.ts <<'EOF'
const count: number = "three";
console.log("running with count =", count);
EOF
echo "--- deno run"
npx --yes deno@2 run main.ts
echo "--- deno run --check"
npx --yes deno@2 run --check main.ts 2>&1; echo "exit: $?"
echo "--- deno check (no run)"
npx --yes deno@2 check main.ts 2>&1 | grep -E "^(TS|error)"

--- deno run
running with count = three
--- deno run --check
Check main.ts
TS2322 [ERROR]: Type 'string' is not assignable to type 'number'.
const count: number = "three";
      ~~~~~
    at file:///tmp/work/main.ts:1:7

error: Type checking failed.

  info: The program failed type-checking, but it still might work correctly.
  hint: Re-run with --no-check to skip type-checking.
exit: 1
--- deno check (no run)
TS2322 [ERROR]: Type 'string' is not assignable to type 'number'.
error: Type checking failed.

Without the flag the program runs and prints the wrong value. With --check, nothing of the program runs: the type error is reported and the exit code is non-zero, so a script or CI job stops.

Type checking per subcommand

Values of the flag

• --check type-checks local modules, the files of your project.
• --check=all also checks remote modules: JSR, npm: packages and URL imports.
• --no-check (on commands that check by default) skips it; --no-check=remote checks local files only.

The compiler options come from compilerOptions in deno.json, so strict and the lib settings are the same for deno run --check, deno check and your editor.

In CI and tasks

{
  "tasks": {
    "dev": "deno run --watch --allow-net main.ts",
    "check": "deno check main.ts",
    "start": "deno run --check --allow-net main.ts"
  }
}

A separate deno check step (or deno check **/*.ts) catches errors in every file, while deno run --check only checks the entry point and what it imports. Tasks are explained in deno.json tasks.

Pitfalls

• --check must come before the script: deno run main.ts --check passes --check to your program in Deno.args.
• Type errors inside dependencies are not reported with plain --check; use --check=all when they matter.
• Checking adds start-up time on the first run; Deno caches the result, so later runs of unchanged files are faster.
• Bun and Node.js type stripping behave like deno run without the flag: see run a TypeScript file with Bun.

FAQ