Snapshots

Snapshots pin the serialized form of a value so you notice when it changes. They suit output that is tedious to assert field by field — rendered strings, serialized structures, large arrays.

Basic usage

import { expect, test } from "as-test";

test("renders a report", () => {
  expect(renderReport(data)).toMatchSnapshot();
});

The first run has nothing to compare against, so record the baseline:

ast test --create-snapshots

Subsequent runs compare the serialized value against the stored snapshot and fail on any difference.

Naming and identity

A snapshot is keyed by its file, its suite nesting path, and a name:

• Unnamed snapshots are numbered per suite in source order — #1, #2, …
• Named snapshots use the string you pass: expect(x).toMatchSnapshot("empty-state").

Because the key includes the suite path, the same name used in two different suites refers to two different snapshots. Prefer names once a suite has more than one snapshot — reordering unnamed snapshots renumbers them.

Updating snapshots

Snapshots live under the configured snapshotDir (default .as-test/snapshots). Manage them from the CLI rather than editing files:

Flag	Effect
--create-snapshots	Record snapshots that don't exist yet. Existing ones still must match.
--overwrite-snapshots	Accept the current output as the new baseline on mismatch.
--no-snapshot	Skip snapshot assertions entirely for this run.

ast test --create-snapshots      # add new snapshots
ast test --overwrite-snapshots   # intentional change: re-baseline

The run summary reports snapshot outcomes separately:

Snapshots: 3 matched, 1 created, 0 updated, 0 failed

When to use them

• Good fits: serialized output, formatted strings, large or nested structures, regression guards on rendering.
• Poor fits: values that change every run (timestamps, random data, pointers). Pin those with a function mock or snapshotFn first, or assert the parts that are stable.

Related

• Snapshots & Throws — the matcher.
• Stable Values — freeze a value so a snapshot stays deterministic.