Stage 2#212Coding · Senior~10 min read

Validate API data at the boundary

React + TypeScript Interview Core · TypeScript State Modeling

Parse a transfer response into trusted domain types and return typed errors for malformed data.

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

Static types do not protect you from JSON that arrived over the network.

Foundation

What to ground before answering

Parse a transfer response into trusted domain types and return typed errors for malformed data.

Focus vocabulary: runtime validation, API, domain types.

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

Validate transfer API data at the boundary

Target: 20mNot run

Make the behavior executable before comparing against the model answer.

type ParseResult =
  | { ok: true; value: { id: string; amountMinor: string; status: 'pending' | 'reconciled' } }
  | { ok: false; error: string };

function parseTransfer(input: unknown): ParseResult {
  // TODO: trust nothing from JSON until it is checked.
  return { ok: false, error: 'not implemented' };
}

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 parseTransfer(input: unknown): ParseResult {
  if (!input || typeof input !== 'object') return { ok: false, error: 'not an object' };
  const value = input as Record<string, unknown>;
  if (typeof value.id !== 'string' || value.id.length === 0) return { ok: false, error: 'bad id' };
  if (typeof value.amountMinor !== 'string' || !/^\\d+$/.test(value.amountMinor)) {
    return { ok: false, error: 'bad amount' };
  }
  if (value.status !== 'pending' && value.status !== 'reconciled') {
    return { ok: false, error: 'bad status' };
  }
  return { ok: true, value: { id: value.id, amountMinor: value.amountMinor, status: value.status } };
}

Review

Recall before moving on

What is the one-sentence answer for "Validate API data at the boundary"?
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?