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Stop fixture drift by defining representative email data once in a test-support module, then importing it wherever tests need it. Use a read-only constant for stable examples and a factory that creates a fresh object for tests that mutate data. One canonical owner keeps defaults and shape discoverable; it does not mean every test should share the same mutable object.
Choose one canonical owner for email fixture data
Put the shared definition in a module that serves as the source of truth, such as test-support/email-fixtures.ts. Tests and packages that need the same representative email should import from that module instead of copying similar objects into local test files.
Keep test-only data out of production dependencies unless the application has a deliberate reason to use it. If your project already has an authoritative email schema or application type, derive the fixture type from it where practical; maintaining a separate hand-written type can create another place for the definition to drift.
There is no required directory layout. Vitest’s practice guide says, “There’s no single right way to organize tests, but some patterns scale better than others.” Co-locating support files with tests and placing them in a separate test directory are both reasonable; choose one convention and apply it consistently. See Vitest’s testing practice guide.
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Use constants for stable examples and factories for mutable data
A constant works well when tests only read a standard valid email. Make it deeply readonly so a test cannot accidentally change a nested field and affect another test. When a test needs to edit the object, call a factory instead so it receives a new value.
// test-support/email-fixtures.ts
export type EmailFixture = {
from: { name: string; address: string };
to: string[];
subject: string;
text: string;
};
export const validEmail: Readonly<EmailFixture> = {
from: { name: "Alice Example", address: "alice@example.com" },
to: ["bob@example.net"],
subject: "Fixture message",
text: "This is example email content.",
};
export function makeEmail(
overrides: Partial<EmailFixture> = {},
): EmailFixture {
return {
...validEmail,
...overrides,
from: { ...validEmail.from, ...overrides.from },
to: overrides.to ? [...overrides.to] : [...validEmail.to],
};
}The factory copies the nested sender object and recipient array as well as the outer object. That matters because a shallow copy alone would still let one test mutate nested data used as a default elsewhere. Extend the copying strategy if the real fixture has more nested mutable fields.
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Use the constant for read-only assertions and the factory for deliberate variation:
import { makeEmail, validEmail } from "../test-support/email-fixtures";
expect(validEmail.from.address).toBe("alice@example.com");
const email = makeEmail({ subject: "Updated subject" });
email.to.push("carol@example.org");Give each test the lifetime it needs
A canonical module can own the shape and defaults without owning one mutable runtime object for the whole suite. Create fresh values per test when tests may modify them. This prevents order-dependent failures in which one test’s changes leak into another.
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import { test as base } from "vitest";
import { makeEmail } from "./email-fixtures";
export const test = base.extend({
email: async ({}, use) => {
await use(makeEmail());
},
});Use the default test scope for values that should be fresh per test. Vitest also documents file- and worker-scoped fixtures; choose those only when setup genuinely needs to live for that scope. Avoid mutable worker-scoped email data if tests can override or mutate it. Consult the Vitest test-context fixture documentation and check the API against the version installed in your project.
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Keep example addresses safely fictional
Use reserved example domains in documentation-style fixtures, such as alice@example.com or bob@example.net. RFC 6761 identifies example.com, example.net, example.org, example, and their subdomains as examples for documentation. For a test specifically about invalid domain names, .invalid makes the intent clear; RFC 2606 also recommends .test for testing.
Reserved names do not disable an application’s mail-sending behavior. If a test must have no external side effects, mock or otherwise control the sender rather than relying on the address alone. See Nodemailer’s SMTP testing guidance.
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| Pattern | Use it when | Key concern |
|---|---|---|
| Readonly constant | Tests need a stable example and do not mutate it. | Protect nested fields too, or a test may still alter shared data. |
| Factory function | Tests need to customize or mutate email values. | Return fresh objects and copy nested mutable values. |
| Vitest test-scoped fixture | Many tests need generated setup exposed through a custom test object. | Prefer per-test lifetime for mutable values. |
| File- or worker-scoped fixture | Setup genuinely needs to be shared at that lifetime. | Shared mutable data can make outcomes depend on execution order. |
When several packages use the same fixture owner, place it where those packages can depend on it without making production code import test-only data. If a package boundary or test runner makes that impractical, retain one authoritative definition and expose it through an appropriate test-support package rather than copying it into each package.
Run type checking separately from tests
A passing Vitest run does not necessarily mean test TypeScript has been type-checked. Vitest transforms TypeScript to execute tests, but its normal run does not perform full type checking. Run the project’s TypeScript checker separately, typically tsc --noEmit, or use Vitest’s type-checking command when configured and supported by the installed version. See Vitest’s TypeScript type-checking guidance.
Keep the test command and type-check command explicit in your package scripts or CI pipeline. The exact scripts and configuration depend on the repository; confirm that CI runs both if fixture types are meant to be enforced.
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