Today I learned@Cybertec

Posts tagged "generics"

Typescript generic with union

Click here to see the initial setup 
enum MessageTypes {
  Success,
  DuplicateEmail,
}

interface ContentMapping {
  [MessageTypes.Success]: {
    data: any;
  };
  [MessageTypes.DuplicateEmail]: {
    email: string;
  };
}

interface Message<Type extends MessageTypes> {
  type: Type;
  content: ContentMapping[Type];
}

We'll compare the following two types:

type Option1 = Message<MessageTypes.DuplicateEmail | MessageTypes.Success>;

type Option2 =
  | Message<MessageTypes.DuplicateEmail>
  | Message<MessageTypes.Success>;
Let's substitute Message and ContentMapping 
type Option1 = {
  type: MessageTypes.DuplicateEmail | MessageTypes.Success;
  content:
    | {
        data: any;
      }
    | {
        email: string;
      };
};

type Option2 =
  | {
      type: MessageTypes.DuplicateEmail;
      content: {
        email: string;
      };
    }
  | {
      type: MessageTypes.Success;
      content: {
        data: any;
      };
    };

In Option1, type and content are not connected. A malformed Message can be created:

const message: Option1 = {
  type: MessageTypes.DuplicateEmail,
  content: {
    data: "This should not be allowed!",
  },
};

On the other hand, Option2 throws an error:

const message2: Option2 = {
  type: MessageTypes.DuplicateEmail,
  content: {
    data: "This *is* not allowed!",
  },
};

Property 'email' is missing in type '{ data: string; }' but required in type '{ email: string; }'.

Check out this TypeScript playground for a live demo.