Testing

meow has a test runner built in. Each test file runs in its own deterministic V8 isolate, so suites can’t leak state into each other and every run is reproducible by construction.

meow test

Writing tests

Import the API from the meow:test built-in module. There’s nothing to install.

math.test.ts

import { test, expect } from "meow:test";

test("adds", () => {
  expect(2 + 2).toBe(4);
});

test("builds the right shape", () => {
  expect({ ok: true }).toEqual({ ok: true });
});

test("throws on bad input", () => {
  expect(() => JSON.parse("{")).toThrow();
});

Discovery

meow test walks the project and runs every file whose name ends in .test. or .spec. with a JS/TS extension (.ts, .tsx, .js, .jsx, .mts, .mjs, .cts, .cjs). It skips node_modules, target, vendor, and hidden directories.

Matchers

The expect() API covers the essentials, each with a .not inverse:

Matcher	Passes when
`toBe(v)`	`Object.is(actual, v)` (identity / primitive equality)
`toEqual(v)`	deep structural equality
`toBeNull()`	value is `null`
`toBeDefined()`	value is not `undefined`
`toBeTruthy()` / `toBeFalsy()`	truthiness
`toThrow()`	calling the function throws
`.not`	inverts any of the above

expect(user.name).not.toBe("");

Output

math.test.ts
  ✓ adds
  ✓ builds the right shape
  ✓ throws on bad input
╭─ meow test ─────────────────────────────────╮
│ 🐾 3 passed · 1 file                          │
╰──────────────────────────────────────────────╯

A failure shows the assertion and the first stack line, and the runner exits non-zero:

sum.test.ts
  ✗ adds
    expect(5).toBe(4)
╭─ meow test ─────────────────────────────────╮
│ ✗ 0 passed, 1 failed · 1 file                 │
╰──────────────────────────────────────────────╯

Determinism is the default

Tests always run in a deterministic strict-web isolate, no matter your project mode. The clock is frozen, randomness is seeded, and the timezone is pinned — so time- and random-dependent tests are stable instead of flaky. If a test genuinely needs real time or host access, that’s a deliberate grant, not the default.

Because each file gets a fresh isolate, there’s no shared global state to reset between files and no cross-test contamination.

The meow:test API

Full type signatures for test() and expect().

Get started

Core concepts

Runtime

Package manager

Toolchain

Observability

Configuration

Writing tests

Discovery

Matchers

Output

Determinism is the default

The meow:test API

​Writing tests

​Discovery

​Matchers

​Output

​Determinism is the default

The meow:test API

Writing tests

Discovery

Matchers

Output

Determinism is the default