20. What are conditional types in TypeScript? How can you use them in your code?

Conditional types in TypeScript allow you to create types that depend on a condition. They are used to select one type based on whether a condition is met. The syntax looks like this:

T extends U ? X : Y

Where T is the type being checked, U is the type to compare against, X is the type returned if the condition is met, and Y is the type returned if the condition is not met.

Example:

type IsString<T> = T extends string ? 'Yes' : 'No';

type Test1 = IsString<'hello'>; // 'Yes'
type Test2 = IsString<42>; // 'No'

In this example, if type T is a string, it will return 'Yes', otherwise, it will return 'No'. Conditional types are useful when you want to adjust types based on other types.