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docs/types/conditional.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)
+
+# Conditional Types
+
+TypeBox supports runtime conditional types with the Extends function. This function performs a structural assignability check against the first (`left`) and second (`right`) arguments and will return either the third (`true`) or fourth (`false`) argument based on the result. The conditional types Exclude and Extract are also supported. The following shows both TypeScript and TypeBox examples of conditional types.
+
+```typescript
+// Extends
+const A = Type.Extends(
+  // type A = string extends number ? 1 : 2
+  Type.String(), //
+  Type.Number(), // ... evaluated as
+  Type.Literal(1), //
+  Type.Literal(2) // const A: TLiteral<2>
+);
+
+// Extract
+const B = Type.Extract(
+  // type B = Extract<1 | 2 | 3, 1>
+  Type.Union([
+    //
+    Type.Literal(1), // ... evaluated as
+    Type.Literal(2), //
+    Type.Literal(3), // const B: TLiteral<1>
+  ]),
+  Type.Literal(1)
+);
+
+// Exclude
+const C = Type.Exclude(
+  // type C = Exclude<1 | 2 | 3, 1>
+  Type.Union([
+    //
+    Type.Literal(1), // ... evaluated as
+    Type.Literal(2), //
+    Type.Literal(3), // const C: TUnion<[
+  ]), //   TLiteral<2>,
+  Type.Literal(1) //   TLiteral<3>,
+); // ]>
+```
+
+Back to [Home](../../readme.md)