boolean is a unionThis is rather a little known fact and common “gotcha” I’d like to share, rather than a full-length text. Let’s just start with an example of a innocent-looking type which wraps a given boolean value:
type Wrap<T> = T extends boolean ? { bool: T } : never;
It seems to work when passing true and false:
type T = Wrap<true>; // { bool: true }
type F = Wrap<false>; // { bool: false }
And also when we pass things that aren’t booleans, like strings:
type X = Wrap<"true">; // never
All is well until we decide to pass boolean:
type B = Wrap<boolean>; // { bool: false } | { bool: true }
Which reveals the true nature of boolean; it’s actually a union type under the hood.
Yes, internally, the boolean type is represented as true | false.
Essentially, it’s shorthand (syntactical sugar, even) for the type true | false.
If you don’t understand what just happened, see distributive conditional types in the official documentation. That’s all for this note, for now.