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docs/types/mapped.md

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+[Overview](../overview.md) | [Installation](../installation.md) | [Usage](../usage.md) | **Types** | [Values](../values/) | [Syntax](../syntax/) | [TypeRegistry](../type-registry.md) | [TypeCheck](../type-check.md)
+
+# Mapped Types
+
+TypeBox supports mapped types with the Mapped function. This function accepts two arguments, the first is a union type typically derived from KeyOf, the second is a mapping function that receives a mapping key `K` that can be used to index properties of a type. The following implements a mapped type that remaps each property to be `T | null`.
+
+```typescript
+const T = Type.Object({
+  // type T = {
+  x: Type.Number(), //   x: number,
+  y: Type.String(), //   y: string,
+  z: Type.Boolean(), //   z: boolean
+}); // }
+
+const M = Type.Mapped(Type.KeyOf(T), (K) => {
+  // type M = { [K in keyof T]: T[K] | null }
+  return Type.Union([Type.Index(T, K), Type.Null()]); //
+}); // ... evaluated as
+//
+// const M: TObject<{
+//   x: TUnion<[TNumber, TNull]>,
+//   y: TUnion<[TString, TNull]>,
+//   z: TUnion<[TBoolean, TNull]>
+// }>
+```
+
+Back to [Home](../../readme.md)