The built-in Record type in TypeScript allows you to create an object with limited properties,
requiring each of the properties exist on the object.
type Properties = "breakfast" | "lunch" | "dinner";
type Hybrid = [name: string, dominant: string, recessive: string];
const preferences: Record<Properties, Hybrid | string> = {
breakfast: "blueberry",
lunch: "raspberry",
dinner: ["marionberry", "chehalem", "olallie"]
}
console.log(preferences.lunch); // "raspberry"
TypeScript handles this just fine. I can assign any string, or any Hybrid to any of the
preexisting Properties. But I cannot assign to a property which does not exist:
const broken_prefs: Record<Properties, Hybrid | string> = {
breakfast: "blueberry",
lunch: "raspberry",
dinner: ["marionberry", "chehalem", "olallie"]
snack: "strawberry"
} // Type '{ snack: string; }' is not assignable to type 'Record<Properties, string | Hybrid>'.
This error is not exactly clear as to what has gone wrong, but the compiler is trying to communicate
that snack is not in Properties. Adding snack to that array would clear things up for
broken_prefs, but we’d also need to add snack to preferences.