25. What is the purpose of infer in TypeScript, and how is it used in advanced type manipulation?

infer is a keyword in TypeScript used in the context of conditional types. It allows you to extract a type from a given expression when the condition is met. It is mainly used in advanced type manipulation, such as extracting types from functions, arrays, or objects.

Example of using infer:

type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

function add(a: number, b: number): number {
  return a + b;
}

type AddReturnType = ReturnType<typeof add>; // 'number'

In this example, infer allows us to extract the return type of a function (in this case, number). Using infer in conditional types enables flexible manipulation of types at the syntactical level, allowing for the creation of more advanced and dynamic types.