Very simple implementation of an Either.
Either represents the possibility of two states, which are correct or error.
In Typescript/Javascript you can throw errors, but errors are somewhat unchecked (means they are not checked at transpile-time) and someone needs to catch them; otherwise they'll bubble up and make your program crash. Sometimes, you are not even aware that a method might throw an error (e.g., because it's bubbling up from another method) so you might not even think about surrounding your code with a try/catch block.