deno test: run tests in Deno and filter a single test

deno test finds the test files in the current folder and its subfolders, type-checks them and runs every Deno.test() case, then prints a summary and exits with code 1 if any test failed. No test framework has to be installed: the runner, assertions (@std/assert from JSR) and coverage are part of Deno. deno test --filter "name" runs only the tests whose name matches.

Write and run tests

cat > math.ts <<'EOF'
export const add = (a: number, b: number): number => a + b;
EOF
cat > math_test.ts <<'EOF'
import { add } from "./math.ts";

Deno.test("add returns the sum", () => {
  if (add(1, 2) !== 3) throw new Error("expected 3");
});

Deno.test("add handles negatives", () => {
  if (add(-2, 2) !== 1) throw new Error("expected 1");
});
EOF
export NO_COLOR=1
npx --yes deno@2 test 2>&1 | sed -E 's/ \([0-9.]+(µs|ms|s)\)//; /^    at /d'
exit "${PIPESTATUS[0]}"

Check math_test.ts
running 2 tests from ./math_test.ts
add returns the sum ... ok
add handles negatives ... FAILED

 ERRORS

add handles negatives => ./math_test.ts:7:6
error: Error: expected 1
  if (add(-2, 2) !== 1) throw new Error("expected 1");
                              ^

 FAILURES

add handles negatives => ./math_test.ts:7:6

FAILED | 1 passed | 1 failed

error: Test failed

The second test is wrong on purpose. Deno prints each test with ok or FAILED, then an ERRORS section with the thrown error and its location, a FAILURES list and the totals; the command exits with code 1. The example runs Deno through npx deno@2 so it needs only Node.js; with Deno installed the command is just deno test.

Run one test with --filter

cat > math_test.ts <<'EOF'
Deno.test("add returns the sum", () => {});
Deno.test("add handles negatives", () => {});
Deno.test("subtract", () => {});
EOF
export NO_COLOR=1
npx --yes deno@2 test --filter "negatives" 2>&1 | sed -E 's/ \([0-9.]+(µs|ms|s)\)//'

Check math_test.ts
running 1 test from ./math_test.ts
add handles negatives ... ok

ok | 1 passed | 0 failed | 2 filtered out

Tests that do not match are counted as filtered out, not as passed. Use --filter "/^add/" for a regular expression.

Common options

Pitfalls

• Permissions. A test that reads a file or opens a socket fails with NotCapable unless you pass the matching --allow-* flag; see Requires read access.
• Type errors stop the run. deno test type-checks local test files first, so a type error fails before any test runs. --no-check skips it, at the cost of catching those errors later.
• Leaks. Deno checks that each test closes the timers, files and network resources it opens, and fails the test with a leak report when it does not. Await your promises and close what you open.
• File names. A file called math.spec.ts is not picked up automatically; name it math_test.ts or math.test.ts, or pass it explicitly.

How to confirm it worked

The last line shows ok or FAILED with the number of passed, failed and filtered tests, and echo $? after the command prints 0 only when every test passed. In CI, that exit code is all you need. Format and lint checks fit in the same job: deno fmt --check and deno lint.

FAQ