Pure Business Logic Unit Tests TaskFlow - Full Stack Developer 2026
Extract deterministic rules first The easiest test is often enabled by the best code shape. A pure policy function accepts values and returns/throws based only on those values. No Nest module, no database, no HTTP, no global clock.
Status transition example export function assertTaskStatusTransition( from: TaskStatus, to: TaskStatus, ) { const allowed = ALLOWED_TRANSITIONS[from] if (!allowed.includes(to)) { throw new TaskStatusTransitionNotAllowed(from, to) } }
describe("assertTaskStatusTransition", () => {
it.each([
["TODO", "IN_PROGRESS"],
["TODO", "CANCELED"],
["IN_PROGRESS", "DONE"],
["DONE", "IN_PROGRESS"],
])("allows %s -> %s", (from, to) => {
expect(() => assertTaskStatusTransition(from, to)).not.toThrow()
})it("rejects DONE -> TODO", () => { expect(() => assertTaskStatusTransition("DONE", "TODO")) .toThrow(TaskStatusTransitionNotAllowed) }) })
Time should be injectable • Avoid new Date() buried inside a rule if the rule needs deterministic testing.
• Pass now into the policy or inject a Clock abstraction only when multiple services genuinely need it.
• Use fake timers for code that is actually timer-driven, not as a default for ordinary date comparisons.
Tests are executable behavioral contracts. Good tests protect meaningful invariants and failure cases, not implementation trivia.
Restate the goal of “Pure Business Logic Unit Tests” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: behavior contract → arrange → act → assert → regression signal. Mark the one boundary this lesson changes.
Inspect the current repository state and record the exact files, route, table, or runtime involved. Avoid creating an abstraction before you can name the existing owner.
Implement the smallest vertical slice. Add one coherent change, run the narrowest verification command, and read the output before continuing.
Connect the slice to TaskFlow’s phase artifact: Vitest + integration + Playwright test suite with deterministic fixtures and CI-ready commands.. Verify behavior from the outside as a user or API client would.
Create one controlled failure that is safe to reproduce. Predict the symptom first, capture evidence, locate the earliest broken boundary, then fix only that cause.
Close the reference and rebuild or explain the core change from memory. Record what you would monitor or test before trusting the same change in production.
it("rejects deleting another member's task", async () => {
const response = await asUser(bob).delete(`/tasks/${aliceTask.id}`);
expect(response.status).toBe(403);
expect(await taskStillExists(aliceTask.id)).toBe(true);
});Do not copy this as a finished answer. Predict what each boundary does, type the smallest useful slice, run it, then compare the result with your prediction.
Look for shared database state, timers, globals, unordered execution, or missing cleanup before adding retries.
Move the highest-risk boundary to an integration test with the real adapter/database.
Without copying the final source, create a second small scenario that applies “Pure Business Logic Unit Tests” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Test a service with controlled dependencies and assert business outcome rather than private calls.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Shared mutable test data causing order-dependent failures
Predict the symptom before causing the failure. Then locate the earliest boundary where reality diverges from expectation.Why does this responsibility live here, what input does it trust, what output proves success, and what would go wrong if another layer owned it?
If your explanation is vague, repeat the boundary trace before moving on.Close the reference. Reproduce the core idea from memory or explain the exact request/data flow aloud. Then check your diff or notes for accidental shortcuts that bypass validation, authorization, error handling, or verification earned in earlier phases.