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Core

define
lazy
equals
logic
parse
assert
nullish
key
primitive
object
predicate
types

Combinators

arrayOf
tupleOf
setOf
mapOf
discriminatedUnion
oneOf
recordOf
struct
typedStruct
oneOfValues

Guides

Filter nullish values
Sync type guards
Compiler API (advanced)
Validate unknown
Validate Hey API types
Refine existing properties
Recursive type guards

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Practical Guides

Start with a real TypeScript problem. Each guide explains the tradeoffs, builds a focused solution, and links to the relevant API reference.

Filter null and undefined from arrays in TypeScript

Remove nullish array entries without losing TypeScript narrowing or valid falsy values.

Keep hand-written type guards in sync with TypeScript types

Make missing, extra, and incompatible string-keyed guard fields visible when an existing object type changes.

Advanced property refinement with TypeScript Compiler API

Apply reusable property refinement to broad AST nodes without treating the TypeScript 7 AST as a discriminated union.

Validate unknown without a schema library

Validate untrusted TypeScript values with small composable guards and a lightweight tagged result.

Validate Hey API generated types with typedStruct

Keep generated response types as the source of truth while validating selected runtime payloads with small hand-written guards.

Refine properties on existing TypeScript types

Build reusable predicates that preserve required, optional, indexed, nested, and literal property refinements.

Recursive type guards in TypeScript with lazy

Define, compose, and safely apply recursive type guards to tree-shaped TypeScript and JSON data.