Test Suite Refactoring

Test Suite Refactoring in iTwin.js

Use this skill when a test file has grown into a monolith whose tests share and mutate the same iModels, creating ordering dependencies. It covers how to split it safely, how to prove you did not regress runtime, and the repo-specific traps that waste the most time.

When this applies

Look for these smells:

• A single describe with dozens of tests and a handful of let imodel1..imodelN declared at suite scope
• iModels opened once in before and mutated by individual tests
• Tests that only pass in declaration order, or fail when run with .only
• Cleanup that closes iModels in after but leaks them if a test throws midway

The hybrid isolation model

Do not give every test its own copy — that is correct but slow. Do not share one mutable iModel — that is fast but fragile. Split by whether the test mutates:

A read-only handle is safe to share precisely because nothing can mutate it. This keeps the common case cheap while making mutation leaks impossible.

Building a read-only fixture

There is no "open a seed read-only" primitive that also isolates you — opening the seed asset directly shares the real repo asset across the whole run. Instead: copy, close, reopen read-only.

// in before()
testBimReadonly = await openReadonlySeedCopy("elements-test.bim", "test.bim", {
  importTestBim: true,
});
compatibilityReadonly = await openReadonlySeedCopy(
  "elements-CompatibilityTestSeed.bim",
  "CompatibilityTestSeed.bim",
);

Tracking mutable iModels

Every writable copy goes through the tracker so teardown is guaranteed even when a test throws.

const { trackMutableIModel, closeTrackedIModels } =
  createMutableIModelTracker();

afterEach(() => closeTrackedIModels()); // idempotent — safe to also call from after()

it("should insert a DisplayStyle", () => {
  const imodel = trackMutableIModel(
    createIModelFromSeed(
      "views-insert-display-style.bim",
      "CompatibilityTestSeed.bim",
    ),
  );
  // ...
});

Type the tracker to IModelDb, not SnapshotDb, so it also accepts StandaloneDb and BriefcaseDb. SnapshotDb and StandaloneDb are siblings (StandaloneDb extends BriefcaseDb extends IModelDb), so a helper typed to SnapshotDb silently excludes standalone tests.

Keep canonical helpers in exactly one place

Every shared fixture helper belongs in a single core/backend/src/test/imodel/IModelTestFixtures.ts next to the tests. Reviewers on this repo push back hard on duplication here — the same boilerplate appearing in 4-6 files defeats the point of having the module.

Helpers proven useful in the reference refactor:

Two design rules that came directly out of review:

• Prefer composition over a boolean flag when the variants differ by an async step. Merging a sync creator and an async creator behind { importTestBim?: boolean } forces every call site async, including the many that are genuinely synchronous. A separate importTestBim(db) keeps sync tests sync and makes the schema import visible at the call site.
• Name helpers so the mechanism is obvious. track → trackMutableIModel, closeTracked → closeTrackedIModels. These names appear at dozens of call sites and are the main thing a newcomer reads.

Before adding a helper, check it is actually reachable. An exported-but-uncalled helper is dead code that reviewers will flag, and deleting it is usually right — but confirm the intended caller is not simply using a subtly different behavior (for example, sharing the seed asset directly versus an isolated copy).

Error helpers must not swallow

A helper that converts a rejection into a value must rethrow anything unexpected, or a real bug reports as a useless assertion message.

export async function getIModelError<T>(
  promise: Promise<T>,
): Promise<IModelError | undefined> {
  try {
    await promise;
    return undefined;
  } catch (err) {
    if (err instanceof IModelError) return err;
    throw err; // a TypeError here is a real bug — keep its message and stack
  }
}

Returning undefined for both "resolved" and "threw the wrong type" collapses two very different failures into expected undefined to not be undefined. After the rethrow, undefined means exactly one thing, and the paired assertion can report the resolved case precisely.

Determinism

Replace Math.random() selection with a deterministic stride so failures reproduce. But check what the randomness was incidentally covering first — sampling with replacement passes duplicate entries, which may be the only coverage of duplicate-input handling. Preserve that intentionally rather than dropping it: keep the deterministic subset and add a deliberate duplicate entry.

Rename the test once selection is deterministic; leaving "random" in the name is misleading.

Naming and file layout

• One file per feature area: IModel<Area>.test.ts (PascalCase, no dots) — for example IModelElements.test.ts, IModelViews.test.ts
• Shared fixtures: IModelTestFixtures.ts in the same folder
• Only prefix files that are genuinely facets of the split suite. Independent subjects already in the folder (Code.test.ts, ProjectExtents.test.ts) should not be renamed to match.

Baseline performance before you start

Splitting a suite multiplies iModel copies, so measure first or you cannot defend the result.

# from core/backend — run 3x before and 3x after
rushx test "lib/cjs/test/imodel/IModel*.test.js" --no-config --require source-map-support/register `
  --timeout 999999 --reporter json --reporter-option output=baseline-run-1.json

Write the JSON with --reporter-option output=<file>, not 1> file.json. rushx prints its own banner (Rush Multi-Project Build Tool ..., the echoed command) to stdout, so redirecting stdout produces a file that is not valid JSON.

Read stats.duration from each run and compare medians. Expect a modest increase from added isolation; treat more than 20% as needing justification. The reference refactor landed at roughly +10% (20.2s to 22.2s) for 80 tests.

Verification loop

# from core/backend
rushx lint          # lints all of ./src/**/*.ts — rushx lint-fix to auto-fix
rushx build:cjs     # tsc --outDir lib/cjs
rushx test "lib/cjs/test/imodel/IModel*.test.js" --no-config --require source-map-support/register --timeout 999999

Confirm the test count is unchanged after a pure split. A drop means a test was lost in the move.

Gotchas that cost the most time

• rushx only runs package.json scripts, not arbitrary binaries. rushx eslint ... fails with "the command is not defined in the package.json file for this project". It does forward extra arguments to the underlying script, which is why rushx test <spec> --reporter json works — it becomes mocha <spec> --reporter json. There is no script that lints a subset, so rushx lint always covers the whole package.
• Tests run against compiled output. Mocha runs lib/cjs/test/..., not src. Always build before running or you are testing stale code.
• Run mocha sequentially. Parallel runs contend on the backend profile lock.
• tsc emits on type errors by default. Tests can pass while the build reports errors, which masks stale-dependency problems. Do not treat a green test run as proof the build is clean.
• Native addon drift produces nonsense failures. A stale @bentley/imodeljs-native surfaces as an assertion failure such as expected undefined to equal 2 — not a dependency error — because the older BisCore schema lacks a property the test expects. When a newly merged test fails inexplicably, compare core/backend/package.json against what is installed, then rush update and rush build --to @itwin/core-backend.
• Do not call shutdownBackend() in a per-file after. TestUtils starts the backend once globally; tearing it down per file is a large regression. Only restart when a file genuinely needs different startup options (such as loadGcsWorkspaces: true), and restore the default afterwards.
• cspell is editor-only here (.vscode/cSpell.json), not part of lint or CI, so // spell-checker: disable directives in test files are noise.

Change management

A pure test refactor touches no shipped API, so it needs no rush extract-api. However, rush change is still required. For pure testing related PRs, the change comment should be left blank. If the refactor pulls in a dependency manifest change (such as a native addon bump arriving via merge), that does require the Rush update flow and a regenerated lock file.

Related skills

• merge-conflict-resolving — handling the modify/delete conflicts that recur while a long-lived refactor branch tracks master
• pr-review-considerations — the review heuristics this repo applies to such PRs