TypeScript and the runtime

What you'll learn

• Where types are honest and where they're a hopeful claim
• Writing type guards and predicates that actually narrow
• Using assertion functions to fail loud at the boundary
• Defining a schema once and inferring the type from it
• Validating fetch responses without lying with `as`
• Storing and migrating data in localStorage safely

1. 01
   The trust boundary

   Where types are honest, and where they are a hopeful claim about runtime values.
   13 min
2. 02
   Type guards and predicates

   Custom checks that teach the compiler what your code already knows.
   15 min
3. 03
   Assertion functions

   Failing loud at the boundary instead of returning a boolean.
   13 min
4. 04
   Schema validation with Zod

   One schema as the source of truth for both runtime checks and TypeScript types.
   18 min
5. 05
   Schema validation alternatives

   Valibot, ArkType, and when Zod is the wrong default.
   13 min
6. 06
   Typing fetch responses

   Why `as User` is a lie, and how to validate the data you actually got.
   16 min
7. 07
   Local storage and untrusted persistence

   Yesterday's data is a stranger. Versioning and migrations on read.
   16 min