Stage 4#402Coding · Senior~10 min read

Write MSW handlers for transfer lifecycle states

Testing + Tooling Drills · API + Mock Boundaries

Mock success, slow pending, backend empty while local pending exists, retryable error, and reconciliation completion.

Prompt

This is a hands-on rep. Attempt the drill before reading the model answer, then narrate the tradeoffs as if an interviewer is watching.

💡

Tip

The mock scenarios should map directly to user-visible states.

Foundation

What to ground before answering

Mock success, slow pending, backend empty while local pending exists, retryable error, and reconciliation completion.

Focus vocabulary: MSW, testing, fixtures.

The useful mental model is not to memorize a perfect answer. It is to explain what owns the data, what can fail, what the user sees, and what test would prove the behavior.

Coding drill

Map transfer scenarios to mock responses

Target: 15mNot run

Make the behavior executable before comparing against the model answer.

type Scenario = 'success' | 'slow-pending' | 'backend-empty' | 'retryable-error' | 'reconciled';
type MockResponse = { status: number; body: { state: string; retryAfterMs?: number } };

function transferMockResponse(scenario: Scenario): MockResponse {
  // TODO: return the HTTP status and body a UI test should see.
  return { status: 200, body: { state: 'unknown' } };
}

TypeScript · runnable

System design

Interview explanation prompt

What problem is this practice item really testing?
What state or contract boundary must be explicit?
What edge case would cause a production regression?
What would you test first?
How would you explain the tradeoff in two minutes?

Self-grade

Strong answer handles the edge cases before polishing syntax.
Strong answer explains why the chosen type or function boundary prevents bugs.
Weak answer passes only the happy path or hides uncertainty in booleans and nullable fields.

Model Answer

function transferMockResponse(scenario: Scenario): MockResponse {
  switch (scenario) {
    case 'success':
      return { status: 202, body: { state: 'submitted' } };
    case 'slow-pending':
      return { status: 200, body: { state: 'pending', retryAfterMs: 1000 } };
    case 'backend-empty':
      return { status: 404, body: { state: 'not-indexed-yet', retryAfterMs: 1000 } };
    case 'retryable-error':
      return { status: 503, body: { state: 'retryable-error', retryAfterMs: 2000 } };
    case 'reconciled':
      return { status: 200, body: { state: 'reconciled' } };
  }
}

Review

Recall before moving on

What is the one-sentence answer for "Write MSW handlers for transfer lifecycle states"?
Which real experience from PR TIMES, React/TypeScript migration, or systems work supports it?
What edge case would you volunteer before the interviewer asks?
What is the smallest test or artifact that proves the design works?