Advanced TypeScript Patterns
Type-Safe State Machines
Model valid state transitions using discriminated unions and state machine maps.
Last Updated: July 29, 2026
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10 min read
Finite State Machines (FSMs) constrain state transitions. TypeScript guarantees that invalid actions for a current state cannot be triggered.
1. Introduction & Architecture
2. Deep Dive & Core Concepts
State machines map current state + event to target next state using discriminated union state definitions.
3. Basic Code Example
4. Advanced Production Patterns
5. Interactive Code Playground
console.log("Type-Safe State Machines prevent illegal state transitions.");6. Common Pitfalls & Edge Cases
Note: Representing states as discriminated unions ensures payload properties (like
data or error) exist ONLY in valid states.7. Interview Q&A & Quizzes
Q: Why use discriminated unions for state machines in TypeScript?
A: They allow TypeScript to narrow available state payloads based on status property tags automatically.
8. Summary Comparison Table
| State | Allowed Events | Next State |
|---|---|---|
idle | FETCH | loading |
loading | RESOLVE / REJECT | success / error |