Test layout

Mirror src layout

Give every source file a twin. When src holds math.ts, tests hold math.test.ts with the same folder nesting: src/math.ts pairs with tests/math.test.ts, and src/shop/cart.ts pairs with tests/shop/cart.test.ts. A reader finds the test for any module without searching, and a new module reminds its author to add the twin.

The mirror also keeps moves cheap. Rename a source file and its test follows the same path, so no index of test locations needs updating.

Name tests .test.ts

End every test file with .test.ts and the separation stays visible forever. The suffix tells readers and tools apart at a glance: one pattern selects the whole suite, and compiled math.test.js never collides with product math.js. Name the twin after its module, so cart.test.ts always tests cart.ts:

ls tests

math.test.ts

One name, one module, zero detective work.

Compile then run with node

No runner is needed here. Compile the test files with tsc, then run the emitted JavaScript with node. A thrown assert fails the run with a non-zero exit, so silence plus green output means the suite passed:

tsc tests/math.test.ts --outDir dist && node dist/math.test.js

pass: doubles two

tsc prints nothing on success and node runs the compiled test. The check helper from the previous lesson prints its pass line, which proves the whole chain from TS source to green output.

Isolate with fresh imports

Tests share nothing when each file imports the module under test and builds fresh values. Never import one test file from another, and never reuse a live object across files. Shared setup may live in a plain helper module, but test state starts empty in every file:

import assert from "node:assert/strict"; import { add } from "../src/math.js"; const result: number = add(2, 3); assert.equal(result, 5);

The js suffix in the import looks odd inside a ts file, but it names the file Node loads after compile time. Keep the suffix and plain tsc plus node runs stay boring and green.

Next: Fixtures

Article author: Arthur Isaev