What are branded types?

Branded types are a slightly hacky way to uniquely identify a property of some type or to ensure at compile time that a given value meets some additional type criterion.

When are they most useful?

Branded types should probably not be used too much since their usage is not really a use case intended by the language maintainers yet. That being said, there are some use cases where they may be practical.

• String values meeting specific formats
  e.g.: email address, UUID, non-empty (#blank?)
• Specific properties of a given type
  e.g.: non-empty for collections, not-only-whitespace for strings, non-zero for numbers

When are they not so useful?

Since branded types aren't really an intentional and supported feature, they don't really play well with the boundaries of the type system.

If you find yourself kind of re-imlpementing a type hierarchy, make sure everything you implement has a real-world purpose or you may be unnecessarily going down an academic rabbit hole of type correctness, as is possible with any sufficiently strict type system.

Ok, so how do I make one if I really want to?

1. Define the brand.

First, define the brand or property you want to ensure at compile time as an interface with a single read-only property with a unique-ish name that is a unique symbol...

interface UUID extends String {
  readonly UUID: unique symbol;
}

• Using an interface instead of a type can reveal when you have another thing defined with the same name and keeps the hover/completion docs succinct.
• Using a readonly unique symbol makes it so that no other type you could import will satisfy this type.

2. Implement a type guard to validate that brand

function isUUID(candidate: string): candidate is UUID {
  return true; // Use a UUID regular expression to do this correctly...
}

Since the brand has a readonly unique symbol as a property, this can effectively make this type guard the only way to validate that a given value meets the type criteria. Yes, you can as any and do whatever you want but then why use Typescript in the first place..?

3. Use your new brand wherever you want more strictness 🎉

function handleID(id: string | UUID) {
  if (isUUID(id)) {
    // Show something special for an actual UUID.
  } else {
    // Fallback to handling a generic string.
  }
}