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# 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>,
); // ]>
```

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