Mapped Types — Animate | AI Wisdom

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🎞 ANIMATE · Mode 3 of 5 · +100 XP— Mapped Types
📋keyof/in
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keyof/in
[K in keyof T] — iterate over all keys of T💡 Analogy
Like running a forEach over the properties of a type instead of an array's values.
keyof T produces a union of T's keys. [K in keyof T] iterates over each key. T[K] looks up the value type for that key.
Console Output
> keyof User = "id" | "name" | "age"
> FrozenUser: { readonly id: string; readonly name: string; readonly age: number }
●mapped-types.ts
keyof/inL3–5
1interface User { id: string; name: string; age: number; }
2 
3// Readonly — all fields become readonly
4type Readonly<T> = { readonly [K in keyof T]: T[K] };
5type FrozenUser = Readonly<User>;
6// { readonly id: string; readonly name: string; readonly age: number }
7 
8// Optional — all fields become optional (Partial implementation)
9type Optional<T> = { [K in keyof T]?: T[K] };
10 
11// Nullable — add null to all fields
12type Nullable<T> = { [K in keyof T]: T[K] | null };
13 
14// Key remapping with 'as' (TypeScript 4.1+)
15type Getters<T> = {
16  [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K]
17};
18type UserGetters = Getters<User>;
19// { getId: () => string; getName: () => string; getAge: () => number }
20 
21// Filter keys by value type using 'never'
22type PickByValue<T, V> = {
23  [K in keyof T as T[K] extends V ? K : never]: T[K]
24};
25type StringFields = PickByValue<User, string>;
26// { id: string; name: string }   — age excluded (number ≠ string)
26 lines
Timeline — 4 Steps
keyof/in
[K in keyof T] — iterate over all keys of T💡 Analogy
Like running a forEach over the properties of a type instead of an array's values.
keyof T produces a union of T's keys. [K in keyof T] iterates over each key. T[K] looks up the value type for that key.
Console Output
> keyof User = "id" | "name" | "age"
> FrozenUser: { readonly id: string; readonly name: string; readonly age: number }

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[K in keyof T] — iterate over all keys of T

💡 Analogy

Like running a forEach over the properties of a type instead of an array's values.

keyof T produces a union of T's keys. [K in keyof T] iterates over each key. T[K] looks up the value type for that key.

Console Output

> keyof User = "id" | "name" | "age"
> FrozenUser: { readonly id: string; readonly name: string; readonly age: number }

[K in keyof T] — iterate over all keys of T

💡 Analogy

Like running a forEach over the properties of a type instead of an array's values.

keyof T produces a union of T's keys. [K in keyof T] iterates over each key. T[K] looks up the value type for that key.

Console Output

> keyof User = "id" | "name" | "age"
> FrozenUser: { readonly id: string; readonly name: string; readonly age: number }