39. What is TypeScript's Extract utility type and how does it differ from Exclude?

TypeScript's Extract Utility Type

The Extract utility type in TypeScript is used to construct a type by extracting from T all union members that are assignable to U. It is particularly useful when you want to filter out specific types from a union type.

Syntax:

type Extract<T, U> = T extends U ? T : never;

Example:

type Fruit = 'apple' | 'banana' | 'orange';
type Citrus = 'orange' | 'lemon';

type Extracted = Extract<Fruit, Citrus>; // 'orange'

In this example, Extracted will only include 'orange' because it is the only type common to both Fruit and Citrus.

How Does Extract Differ from Exclude?

• Extract: Retains only the types present in both T and U.
• Exclude: Removes types from T that are present in U.

Exclude Example:

type NonCitrus = Exclude<Fruit, Citrus>; // 'apple' | 'banana'

In this case, NonCitrus will include 'apple' and 'banana' because they are not present in Citrus.

When to Use Extract vs. Exclude

• Use Extract when you want to narrow down a union type to only include certain members.
• Use Exclude when you want to filter out certain members from a union type.

Understanding these utility types can help you write more expressive and maintainable TypeScript code, especially when dealing with complex types and type transformations.