Know How to Reconstruct Types at Runtime

Overview

TypeScript types are erased at runtime, but sometimes you need to validate that runtime data matches your types. Use libraries like io-ts, zod, or runtypes to define schemas that provide both runtime validation and static types. This ensures your types match reality.

When to Use This Skill

• Validating data at runtime
• Parsing JSON with type safety
• Types need runtime checks
• Using io-ts, zod, or runtypes
• Building validation schemas

The Iron Rule

Use validation libraries like zod or io-ts to get both runtime validation and TypeScript types from a single source of truth.

Example with Zod

import { z } from 'zod';

// Define schema once
const UserSchema = z.object({
  id: z.string(),
  name: z.string(),
  email: z.string().email(),
  age: z.number().optional(),
});

// Get TypeScript type from schema
type User = z.infer<typeof UserSchema>;

// Runtime validation
const result = UserSchema.safeParse(unknownData);
if (result.success) {
  // result.data is typed as User
  console.log(result.data.name);
} else {
  console.error(result.error);
}

Reference

• Effective TypeScript, 2nd Edition by Dan Vanderkam
• Item 74: Know How to Reconstruct Types at Runtime