Twelve concepts

Type aliases

A name for a type. Everything else here is an alias that takes arguments.

Generic parameters

Aliases take arguments. `extends` in this position is a constraint, not inheritance.

keyof and indexed access

`keyof T` is the union of T’s keys; `T[K]` reads a property type back out.

Mapped types

Build an object by iterating a union of keys: `{ [K in Keys]: Value }`.

Practiced in#212 Rebuild Pick#219 Rebuild Partial#226 Rebuild Readonly#235 PickByType#236 Getters#241 Rebuild Required#242 Merge#246 Entries#247 Omit by type#249 Optional, but only these#256 Invert#257 Tuple to object#258 Which keys are optional

Modifiers

Add or strip `readonly` and `?` while mapping, with `+` and `-`.

Practiced in#212 Rebuild Pick#219 Rebuild Partial#226 Rebuild Readonly#235 PickByType#236 Getters#241 Rebuild Required#242 Merge#246 Entries#247 Omit by type#249 Optional, but only these#256 Invert#257 Tuple to object#258 Which keys are optional

Conditional types

`A extends B ? X : Y` — the type-level `if`, asking about assignability.

Practiced in#220 Rebuild Exclude#230 Permutation#238 UnionToIntersection#248 Is it never?

infer

Bind a type variable inside a pattern and use it in the true branch.

Practiced in#213 Unwrap#221 Return Type

Distributivity

A naked type parameter in a conditional distributes over unions. `[T]` stops it.

Practiced in#220 Rebuild Exclude#230 Permutation#238 UnionToIntersection#248 Is it never?

Template literal types

String patterns at the type level. Combined with `infer`, they parse.

Practiced in#214 Route Params#217 Split#224 Join#225 Trim#228 Replace#231 CamelCase#233 StartsWith#243 Absolute#244 Repeat#252 Query string#253 Snake case

Recursion

A conditional type that calls itself. The false branch is the exit.

Practiced in#215 Deep Readonly#222 Reverse#227 Includes#229 Flatten#237 Unique#240 Filter#245 Paths#251 Optional all the way down#259 Get by path

Variadic tuples

Spread tuples into tuples. `length` turns them into a counter.

Practiced in#216 Type-level Add#218 Last#223 Type-level Subtract#232 Zip#234 TupleToUnion#239 Chunk#250 Fill#254 Without#255 First

Exact equality

Why `Equal<X, Y>` is not `extends`, and why an intersection fails it.