ReddRadar
Agent skill
type-assertions
Type assertion patterns using @kitz/assert. Triggers on type-level tests, .test-d.ts files, or when asserting TypeScript types.
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Install this agent skill to your Project
npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/data/kitz-type-assertions
SKILL.md
Type Assertions
Import
typescript
import { Assert } from '@kitz/assert'
Builder Pattern
Assertions use a fluent builder: Assert.<relator>.<matcher>(expected).<setting>().on(actual)
Relators (Required First)
| Relator | Meaning |
|---|---|
.exact |
Types must be identical |
.equiv |
Bidirectional assignability |
.sub |
Actual extends expected |
Unary Matchers (No Expected Type)
typescript
Assert.exact.any.on(value) // value is any
Assert.exact.unknown.on(value) // value is unknown
Assert.exact.never.on(value) // value is never
Assert.exact.empty.on(value) // value is {}
Binary Matchers
typescript
// Value-level (preferred)
Assert.exact.of(expected).on(actual)
// Type-level only
Assert.exact.ofAs<Expected>().onAs<Actual>()
// Shorthand matchers
Assert.exact.string.on(value)
Assert.exact.number.on(value)
Assert.exact.boolean.on(value)
Assert.exact.bigint.on(value)
Assert.exact.symbol.on(value)
Assert.exact.null.on(value)
Assert.exact.undefined.on(value)
Assert.exact.void.on(value)
Assert.exact.object.on(value)
Assert.exact.function.on(value)
Settings (Optional)
typescript
// Negate assertion
Assert.exact.of(string).not.on(value) // value is NOT string
// Control type inference
Assert.exact.of(expected).inferNarrow().on(actual) // narrow inference
Assert.exact.of(expected).inferWide().on(actual) // wide inference
Assert.exact.of(expected).inferAuto().on(actual) // default
Extractors
typescript
// Extract return type
Assert.exact.string.returned.on(fn) // ReturnType<fn> is string
// Extract awaited type
Assert.exact.string.awaited.on(promise) // Awaited<promise> is string
// Extract parameters
Assert.exact.of([string, number]).parameters.on(fn)
// Extract array element
Assert.exact.string.array.on(arr) // arr is string[]
Preferred Pattern
ALWAYS prefer value-level API - it reports ALL failures simultaneously:
typescript
// In .test.ts files
test('types', () => {
const value = getValue()
Assert.exact.string.on(value)
Assert.exact.of({ a: 1 }).on(obj)
})
// In .test-d.ts files (pure type-level)
Assert.exact.ofAs<string>().onAs<string>()
Assert.exact.ofAs<number>().onAs<number>()
// @ts-expect-error - type mismatch
Assert.exact.ofAs<string>().onAs<number>()
Avoid
typescript
// DON'T use type-level only assertions in test blocks
test('bad', () => {
type _ = Assert.exact.of<string, number> // Short-circuits on first error
})
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