Create Explicit and Readable Type Declarations with TypeScript mapped Type Modifiers

Hi, quick question, in which cases we would need to avoid modifying an interface itself and choosing modify it through a type?

Hi, Eduardo! If you can modify your interface directly then I'd recommend going for that!

Mapped type modifiers are useful if you either:

• have an interface you can't modify directly (like one from a library)
• have an interface that you want to continue using for some purposes, and create a slight variation of it (using mapped type modifiers) to use for other purposes

In both cases, mapped type modifiers "follow" the shape of the original interface, even if it changes/gets modified in the future, and just augment it based on the rules you specify.

there's a typo here: readonly [K in key of IPet]: IPet[K]; should be readonly [K in keyof IPet]: IPet[K];