Conditional Types Basics

// Extract the resolved value of a Promise (or leave non-promise types alone).
type Unwrap<T> = T extends Promise<infer U> ? U : T;

type A = Unwrap<Promise<string>>;     // string
type B = Unwrap<number>;              // number

// Extract function argument types as a tuple.
type Args<F> = F extends (...args: infer P) => any ? P : never;

type FirstArg<F> = Args<F>[0];

const greet = (name: string, formal: boolean) => `Hello, ${name}`;
type Greeting = ReturnType<typeof greet>;   // string
type GreetArgs = Args<typeof greet>;        // [string, boolean]
type WhosName  = FirstArg<typeof greet>;    // string