# TypeScript Mapped Types

> Transform object types by iterating over their keys with mapped type syntax

## When to Use

- Creating new types by transforming all properties of an existing type
- Making all properties optional, readonly, or nullable
- Remapping keys to create derived property names
- Building type-safe form state, API response wrappers, or configuration objects

## Instructions

1. **Basic mapped type** — iterate over keys with `in keyof`:

```typescript
type Readonly<T> = {
  readonly [K in keyof T]: T[K];
};

type Optional<T> = {
  [K in keyof T]?: T[K];
};
```

2. **Add or remove modifiers** with `+` and `-`:

```typescript
type Mutable<T> = {
  -readonly [K in keyof T]: T[K];
};

type Required<T> = {
  [K in keyof T]-?: T[K];
};
```

3. **Transform value types:**

```typescript
type Nullable<T> = {
  [K in keyof T]: T[K] | null;
};

type Getters<T> = {
  [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K];
};

type User = { name: string; age: number };
type UserGetters = Getters<User>;
// { getName: () => string; getAge: () => number }
```

4. **Key remapping with `as`:**

```typescript
type PrefixedKeys<T, P extends string> = {
  [K in keyof T as `${P}${Capitalize<string & K>}`]: T[K];
};

type ApiUser = PrefixedKeys<User, 'user'>;
// { userName: string; userAge: number }
```

5. **Filter keys with `as` and `never`:**

```typescript
type OnlyStrings<T> = {
  [K in keyof T as T[K] extends string ? K : never]: T[K];
};

type StringFields = OnlyStrings<{ name: string; age: number; email: string }>;
// { name: string; email: string }
```

6. **Map over a union of string literals:**

```typescript
type EventHandlers<T extends string> = {
  [K in T as `on${Capitalize<K>}`]: (event: K) => void;
};

type Handlers = EventHandlers<'click' | 'hover' | 'focus'>;
// { onClick: (event: 'click') => void; onHover: ...; onFocus: ... }
```

7. **Combine with conditional types:**

```typescript
type DeepPartial<T> = {
  [K in keyof T]?: T[K] extends object ? DeepPartial<T[K]> : T[K];
};
```

8. **Practical patterns:**

```typescript
// Form state with touched/error tracking
type FormState<T> = {
  values: T;
  touched: { [K in keyof T]: boolean };
  errors: { [K in keyof T]?: string };
};

// Pick and transform
type PickAndNullify<T, K extends keyof T> = {
  [P in K]: T[P] | null;
};
```

## Details

Mapped types iterate over a set of keys (from `keyof T` or a string literal union) and produce a new object type with one property per key. They are the foundation of most built-in utility types (`Partial`, `Required`, `Readonly`, `Pick`, `Record`).

**Homomorphic mapped types:** When the key source is `keyof T`, the mapped type preserves optional and readonly modifiers from the original type. This is why `Partial<T>` works correctly on types that already have optional properties.

**Key remapping (TypeScript 4.1+):** The `as` clause in a mapped type lets you transform key names, filter keys (by mapping to `never`), or create entirely new key sets from the original keys.

**Template literal types + mapped types:** Combining `as \`prefix${K}\`` with mapped types creates patterns like getter/setter generation, event handler typing, and API route typing.

**Recursive mapped types:** TypeScript allows recursive mapped types for deep transformations (`DeepPartial`, `DeepReadonly`). The compiler has a recursion depth limit — extremely deep object nesting can cause issues.

**Trade-offs:**

- Mapped types produce clear, readable type transformations but complex mapped types can be hard to debug
- Key remapping with `as` is powerful but the type error messages when it fails are often cryptic
- Recursive mapped types can slow the compiler on deeply nested structures

## Source

https://typescriptlang.org/docs/handbook/2/mapped-types.html

## Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

## Harness Integration

- **Type:** knowledge — this skill is a reference document, not a procedural workflow.
- **No tools or state** — consumed as context by other skills and agents.

## Success Criteria

- The patterns described in this document are applied correctly in the implementation.
- Edge cases and anti-patterns listed in this document are avoided.
