typescript-advanced-type-system

TypeScript Type System

The type-level mechanics that everything else in this skill family builds on: unions, conditionals, mapped types, template literals, generics, and type-level extraction with infer.

Mindset

Model the problem in types before reaching for a runtime check — a type that makes an invalid combination unrepresentable is a compile-time guarantee, while a runtime check for the same thing only protects the paths that get exercised. Prefer deriving a type from another type (ReturnType<typeof fn>, a mapped type, a conditional extraction with infer) over hand-writing a parallel type that can drift out of sync with the thing it describes. When a conditional type distributes over a union unexpectedly, that is usually the type system doing exactly what it's told — wrap the checked type in a tuple ([T] extends [U]) to opt out of distribution rather than restructuring the type to dodge it. A type assertion (as) is a claim, not a check; only reach for one when you have already established the fact some other way (e.g. immediately after a runtime guard), never as a way to silence an error you haven't investigated.

When to Apply

Use this skill when:

• Modeling a union or intersection type, including discriminated unions at the type-definition level
• Writing a conditional type, especially one using infer to extract a nested type
• Building a mapped type or remapping its keys with an as clause
• Constructing a template literal type (e.g. typed route parameters)
• Designing a generic function or type with meaningful constraints
• Extracting a type from a function, array, promise, or constructor
• Deciding whether a value needs an index signature or a fixed shape
• Choosing between a type assertion and a real narrowing check

Use When

• "How do I extract the return type / element type / awaited type from this?"
• "How should I model this union or generic type?"
• "How do I build a type that changes shape based on another type?"
• "What's the difference between as and a type guard here?"