denoland/deno · error · AssertionError

Missing snapshot file.

Error message

Missing snapshot file.

What it means

In check mode (a normal `deno test` run), assertSnapshot reads the snapshot file resolved from options.dir/options.path (defaulting next to the test file). If that file does not exist at all, it throws AssertionError "Missing snapshot file." before any comparison (cli/js/40_test_snapshot.js). In update mode (`deno test --update`) the same situation instead creates the file, which is why this error classically appears on the first check-mode run of brand-new snapshot tests.

Solutions

1. Run `deno test --update` (or `-u`) once to generate the snapshot file, then re-run plain `deno test`.
2. Commit the generated snapshot file to version control so CI and teammates have it.
3. Verify options.dir/options.path resolve to the same location in generate and check runs.
4. Check that .gitignore does not exclude the __snapshots__ directory or *.snap files.

Example fix

# before
deno test   # AssertionError: Missing snapshot file.

# after
deno test --update   # create/refresh snapshots
deno test            # now passes

// Fail with a clear message before tests run if the snapshot file is absent:
import { assert } from "jsr:@std/assert";
const snap = new URL("./__snapshots__/main.spec.snap", import.meta.url);
if (import.meta.main === false && !isFirstRun) {
  assert(Deno.statSync(snap).isFile, `missing snapshot file: ${snap.pathname} — run 'deno test --update'`);
}

Deno.test("snap", async (t) => {
  try {
    await t.assertSnapshot(value);
  } catch (err) {
    if (err instanceof Deno.errors.NotFound || String(err.message).includes("Missing snapshot file")) {
      throw new Error("No snapshot file — run 'deno test --update' first");
    }
    throw err;
  }
});

When it happens

Trigger: Running `deno test` (check mode) on tests that call assertSnapshot for the first time with no snapshot file present; the __snapshots__ file was deleted or never committed; options.dir/options.path point at a directory where no snapshot file exists.

Common situations: New snapshot tests written but never run with --update; the snapshot file missing from version control so CI fails locally-passing tests; a custom dir/path option that does not match where snapshots were generated; .gitignore rules swallowing the snapshots directory.

Related errors

• Expected the second argument to assertSnapshot() to be an…
• Missing snapshot
• Snapshot does not match:\n\n
• Snapshot serializer must return a string
• A file exists at the destination

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/16c602ab9f6457bf. Report an issue: GitHub.

Appendix: source

Thrown at cli/js/40_test_snapshot.js:353

  const actualSnapshot = serializer(actual);
  if (typeof actualSnapshot !== "string") {
    throw new TypeError("Snapshot serializer must return a string");
  }

  const expectedSnapshot = MapPrototypeGet(context.currentValues, name);

  if (getIsUpdateMode()) {
    if (actualSnapshot !== expectedSnapshot) {
      MapPrototypeSet(context.updatedValues, name, actualSnapshot);
      if (!ArrayPrototypeIncludes(context.updatedNames, name)) {
        ArrayPrototypePush(context.updatedNames, name);
      }
    }
    return;
  }

  if (!context.fileExists) {
    throw new AssertionError(
      getErrorMessage("Missing snapshot file.", options),
    );
  }
  if (expectedSnapshot === undefined) {
    throw new AssertionError(
      getErrorMessage(`Missing snapshot: ${name}`, options),
    );
  }
  if (actualSnapshot === expectedSnapshot) {
    return;
  }
  throw new AssertionError(
    getErrorMessage(
      getSnapshotNotMatchMessage(actualSnapshot, expectedSnapshot),
      options,
    ),
  );
}

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