Skip to content
Sarj Code Standards

no-first-party-module-mock

Prefer injected collaborators over mocking maintained first-party modules in tests.

noFirstPartyModuleMock Rule source Tests Catalog JSON

Why

Replacing an owned module couples tests to its lookup path and can verify mock wiring while bypassing the real implementation contract.

Fix

Extract a narrow port or pure core operation and inject a typed fake; retain module mocking only when module lookup or bootstrap behavior is the contract.

Examples

Before — flagged Do not replace an owned module
action.test.ts
import { vi } from "vitest";
vi.mock("./service", () => ({ run: vi.fn() }));
After — preferred Inject a narrow service port
action.test.ts
const result = await runAction({ service: recordingService });
Before — flagged Exercise the maintained ESM implementation — before
tests/service.test.ts
import { jest } from "@jest/globals";
jest.unstable_mockModule("./service.js", () => ({}));
After — preferred Exercise the maintained ESM implementation — after
tests/service.test.ts
import { run } from "./service.js";
run();

Options

{
  "additionalProperties": false,
  "properties": {
    "additionalModulePrefixes": {
      "description": "Additional maintained module prefixes that tests must not mock.",
      "items": { "minLength": 1, "type": "string" },
      "type": "array",
      "uniqueItems": true
    }
  },
  "type": "object"
}

References