Writing Reliable Themis Tests for a Scheduled Job Service

Problem/Feature Description

Vantage Systems runs a scheduled job service that processes batch reports. Three modules need test coverage: a ReportScheduler that coordinates batch report runs using the current timestamp, a ReportAggregator that sums numeric values from a list of records, and a fetchExternalRate function that calls an external currency rate API to get the latest USD/EUR exchange rate.

The QA lead has raised concerns in the past about flaky tests — tests that pass sometimes and fail other times because they depend on the clock, produce random IDs, or make real network calls. The team wants Themis tests authored in a way that avoids this problem.

You've been asked to author Themis tests for all three modules and write a brief note explaining how you handled the nondeterminism risks for each.

Output Specification

Produce the following files:

• __themis__/tests/report-scheduler.test.ts — Themis test for the ReportScheduler that handles its dependency on the current time.
• __themis__/tests/report-aggregator.test.ts — Themis test for the ReportAggregator pure aggregation logic.
• __themis__/tests/fetch-external-rate.test.ts — Themis test for the fetchExternalRate function that handles its network dependency.
• TESTING_NOTES.md — A markdown document explaining how you addressed nondeterminism for each module, and what general approach you took for time-dependent and network-dependent functions.

Input Files

The following files represent the source code to be tested. Extract them before beginning.

=============== FILE: src/report-scheduler.ts =============== export interface ScheduledRun { runId: string; scheduledAt: number; reportType: string; }

export class ReportScheduler { schedule(reportType: string, clock: () => number = Date.now): ScheduledRun { return { runId: run-${clock()}, scheduledAt: clock(), reportType, }; } }

=============== FILE: src/report-aggregator.ts =============== export interface Record { id: string; value: number; }

export function aggregateRecords(records: Record[]): number { return records.reduce((sum, r) => sum + r.value, 0); }

=============== FILE: src/fetch-external-rate.ts =============== export interface RateResponse { base: string; target: string; rate: number; }

export async function fetchExternalRate( base: string, target: string, fetcher: (url: string) => Promise<{ rate: number }> = async (url) => { const res = await fetch(url); return res.json() as Promise<{ rate: number }>; } ): Promise<RateResponse> { const data = await fetcher(https://api.example.com/rates/${base}/${target}); return { base, target, rate: data.rate }; }