Stage 3#302Coding · Senior~10 min read

Make illegal transition states unrepresentable

Operation State Architecture · State Machines

Use TypeScript unions and transition functions so impossible UI states cannot compile.

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

This is where frontend architecture and TypeScript interview skills meet.

Foundation

What to ground before answering

Use TypeScript unions and transition functions so impossible UI states cannot compile.

Focus vocabulary: TypeScript, state machine, exhaustiveness.

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

Make impossible transfer states explicit

Target: 15mNot run

Make the behavior executable before comparing against the model answer.

type TransferState =
  | { tag: 'idle' }
  | { tag: 'validating'; amountMinor: string }
  | { tag: 'ready'; amountMinor: string; recipient: string }
  | { tag: 'submitted'; operationId: string }
  | { tag: 'failed'; operationId: string; retryable: boolean };

function describeTransferState(state: TransferState): string {
  // TODO: exhaustively return the user-facing state label.
  return '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 describeTransferState(state: TransferState): string {
  switch (state.tag) {
    case 'idle':
      return 'edit';
    case 'validating':
      return 'validating';
    case 'ready':
      return 'ready';
    case 'submitted':
      return 'tracking';
    case 'failed':
      return state.retryable ? 'retry' : 'support';
  }
}

Review

Recall before moving on

What is the one-sentence answer for "Make illegal transition states unrepresentable"?
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?