Stage 6#622Coding · Senior~10 min read

Run a 35-minute coding + narration mock

System Design + Mock Loop · Mock Interview Loop

Implement a state reducer, amount parser, or merge function while explaining tradeoffs and tests.

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.

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Tip

Interviewers are listening for judgment while you code.

Foundation

What to ground before answering

Implement a state reducer, amount parser, or merge function while explaining tradeoffs and tests.

Focus vocabulary: mock interview, coding, narration.

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

Implement and narrate a transfer status reducer

Target: 20mNot run

Make the behavior executable before comparing against the model answer.

type Status = 'draft' | 'submitting' | 'pending' | 'reconciled' | 'retryable-error';
type Event = 'submit' | 'accepted' | 'reconciled' | 'timeout' | 'retry';

function reduceStatus(status: Status, event: Event): Status {
  // TODO: implement a small reducer and be ready to narrate each transition.
  return status;
}

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 reduceStatus(status: Status, event: Event): Status {
  if (status === 'draft' && event === 'submit') return 'submitting';
  if (status === 'submitting' && event === 'accepted') return 'pending';
  if (status === 'pending' && event === 'reconciled') return 'reconciled';
  if ((status === 'submitting' || status === 'pending') && event === 'timeout') return 'retryable-error';
  if (status === 'retryable-error' && event === 'retry') return 'submitting';
  return status;
}

Review

Recall before moving on

What is the one-sentence answer for "Run a 35-minute coding + narration mock"?
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?