Super Set Unions

So we know we can drastically narrow a primitive type like "number" by using a union of literal types. For example, maybe only 3 error codes are valid:

type ErrorSlugs = "OK" | "NOT_FOUND" | "INTERNAL_ERROR";

This works great if these are the only valid error codes, but what if:

1. Any string can be used as an error slug
2. "OK", "NOT_FOUND", and "INTERNAL_ERROR" are the most common values and we like to have them show up in autocomplete

TypeScript has a hacky way for us to express this: super set unions.

type ErrorCodes = "OK" | "NOT_FOUND" | "INTERNAL_ERROR" | (string & {});

You might be wondering,

"Why wouldn't I just use string - the set of allowed values is the same?"

And you're right, but there's one subtle difference. By adding (string & {}), TypeScript won't change which values are allowed. Any string is allowed. But it will still give us autocomplete in our editor for the values "OK", "NOT_FOUND", and "INTERNAL_ERROR".