Keyof, in, +/- modifiers, remapping with as, homomorphic types.

Mapped Types — Transform Every Property in a Type

Mapped types iterate over the keys of a type and transform each property. Syntax: type M<T> = { [K in keyof T]: Transform<T[K]> }. The +/- modifier adds or removes ? (optional) and readonly. The as clause (TypeScript 4.1+) remaps key names. Mapped types that take a generic parameter and map over keyof T are homomorphic — they preserve modifiers from the original type. Built-in Partial, Readonly, Required, and Pick are all homomorphic mapped types.

Interactive Pipeline Flow

UnderstandImplementOptimise

📦 Source — keyof T

keyof T produces a union of T's string/number/symbol keys. For interface User { name: string; age: number }, keyof User = "name" | "age".

Architecture Diagrams

Using as clause to generate getter method types.

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Key Design Decisions

🔄Mapped types are the engine of utility types

All of Partial, Required, Readonly, and Pick are 1-4 line mapped types. Learning to write your own mapped types lets you create any type transformation you can imagine.

🔀as clause enables domain-specific type generation

Template literal type keys with as let you generate entire API surface types from a model. Getters<T>, Setters<T>, Validators<T> are all expressible in 3-4 lines of mapped type code.

🔍Filter properties with as never

as never in the key position removes that property from the output type. type StringProps<T> = { [K in keyof T as T[K] extends string ? K : never]: T[K] } keeps only string properties.

Discussion