What are some type-level TypeScript libraries? [on hold]

Question

To learn about the TS ecosystem, I'm looking for examples of type-level TS libraries.

By 'type-level TS library', I mean something like an npm package that does not provide typings for a specific value-level library, but instead provides facilities for type-level programming.

Some examples of this sort of thing:

• fp-ts has higher-kinded-types
• utility-types is 'lodash for types'
• There's a library for testing types using types only (can't remember the name)

What are more examples of type-level TS libraries?

This is an objective question and I'm genuinely trying to learn something. I am not asking for 'good' vs. 'bad', just examples.

Answer 1

Some honorable mentions:

Types

• type-fest A collection of essential TypeScript types
• ts-essentials All basic TypeScript types in one place
• typescript-conditional-types Helpers for typescript generic types
• ts-types-utils Type utilities for typescript
• type-zoo A menagerie of useful type operators for TypeScript
• unionize Boilerplate-free functional sum types in TypeScript
• hkts A simple encoding of higher-kinded types in TypeScript
• safe-units Type-safe TypeScript units of measure
• type-plus Additional types and types adjusted utilities for TypeScript

Test own Types with Types

• is Type check values
• conditional-type-checks Types for testing TypeScript types.

Transformers

• ts-transformer-keys