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Problem 26

Write Tests for an API Client

MEDIUMTEST
TypeScript+4

You're writing tests for an ApiClient class — a reusable wrapper around fetch that handles errors, parses JSON, and retries transient server failures.

The class is in src/ApiClient.ts — read-only. Your job is in tests/ApiClient.test.ts.

The judge will swap in four buggy variants of the class. Your tests must:

  1. All pass against the correct implementation.
  2. Catch each bug — at least one test must fail against each buggy variant.

You can't make real network calls in the judge. Replace globalThis.fetch with a vi.fn() mock before each test.

Requirements
  • Use Vitest (describe, it, expect, vi) — already available in the runtime.
  • Import ApiClient from '../src/ApiClient'.
  • Mock globalThis.fetch using vi.fn().mockResolvedValue(...) — do not make real network calls.
  • Every test must pass against the reference implementation.
  • Your suite must produce at least one failing test against each of four buggy variants: missing error handling on any non-ok response, 4xx treated as success (only 5xx throws), raw Response object returned instead of parsed JSON, and 401 triggering retries when it should throw immediately.
Examples
Example 1
Input
GET /users → 200 with body { id: 1 }
Output
Promise resolves to { id: 1 }
Note

A 200 response is parsed as JSON and returned directly.

Example 2
Input
GET /not-found → 404
Output
Promise rejects with Error('HTTP 404')
Note

Any non-ok status that isn't retried must throw with the status code.

Example 3
Input
GET /protected → 401 (maxRetries = 2)
Output
fetch called exactly once, then rejects with Error('HTTP 401')
Note

Auth errors are not transient — the client must not retry them.

Constraints
  • Do not modify src/ApiClient.ts.
  • All assertions go in tests/ApiClient.test.ts.
  • Use vi.fn() for fetch mocking — no real HTTP calls.
Follow-up

The retry strategy here is a flat counter. Real systems use exponential backoff — each retry waits 2× longer than the last. How would you test that without making your test suite take seconds to run?

Hints
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Problem 22
Write Tests for a Currency Amount Formatter
easy
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