Types are a lot like sets, we can use special type operations to describe unions and intersections of types:
|for union&for intersection
Union
A value(or partial of a value) with a union type | satisfy at least one of a type, it can satisfy all of the types at once:
type Cat = { name: string, purrs: true};
type Dog = { name: string, barks: true, wags: true};
type CatOrDogOrBoth = Cat | Dog;
let cat: CatOrDogOrBoth = { name: 'Bonkers', purrs: true }; // satisfy Cat
let dog: CatOrDogOrBoth = { name: 'Domino', barks: true, wags: true }; // satisfy Dog
let both: CatOrDogOrBoth = { name: 'Donkers', barks: true, wags: true, purrs: true}; // satisfy both: Dog and Cat
let cat2: CatOrDogOrBoth = { name: 'Donkers', barks: true, purrs: true}; // partial of it satisfy Cat
let both3: CatOrDogOrBoth = { name: 'Donkers', barks: true}; // Type '{ name: string; barks: true; }' is not assignable to type 'CatOrDogOrBoth'.
Property 'wags' is missing in type '{ name: string; barks: true; }' but required in type 'Dog'
Another example, if a function returns type Results = string | null, then it might return a string, or it might return a null.
Intersection
A value with a intersection type & must satisfy all of the types at once.
type CatAndDog = Cat & Dog;
let both: CatAndDog = { name: 'Donkers', barks: true, wags: true, purrs: true};