Conditional Types

Conditional types in TypeScript are a way to select a type based on a condition. They allow you to write a type that dynamically chooses a type based on the types of its inputs. Conditional types are declared using a combination of the infer keyword and a type that tests a condition and selects a type based on the result of the test.

For example, the following is a conditional type that takes two types and returns the type of the first argument if it extends the second argument, and the type of the second argument otherwise:

type Extends<T, U> = T extends U ? T : U;

type A = Extends<string, any>; // type A is 'string'
type B = Extends<any, string>; // type B is 'string'

In this example, the Extends conditional type takes two types T and U and returns the type of the first argument T if it extends the second argument U, and the type of the second argument U otherwise. The T extends U syntax is used to test whether T extends U, and the ? T : U syntax is used to select the type T if the test passes and the type U otherwise.

Learn more from the following links:

• Conditional Types
• Conditional Types - Advanced TypeScript