Mapped types

type Fruit = {
  name: string
  color: string
  mass: number
}
 
// mapped type
type MyRecord = { [FruitKey in "apple" | "cherry"]: Fruit }
 
function printFruitCatalog(fruitCatalog: MyRecord) {
  fruitCatalog.cherry
  fruitCatalog.apple
  // ^ (property) apple: Fruit

  fruitCatalog.pineapple
             // ^ Property 'pineapple' does not exist on type 'MyRecord'.
}