How Do Discriminated Unions Enhance TypeScript Safety?

If you enjoy this story, feel free to like or share it with others who might find it helpful!

---

I’m a librarian in a library where each book can transform into different forms. These books are not ordinary; they’re part of a special collection known as “Discriminated Unions.” Each book has a special symbol on its spine, like a unique emblem, that tells me what form it can take—whether it’s a novel, a textbook, or a comic.

In this library, the emblem on the spine is my guiding star. It’s like having a secret code that ensures I always know what kind of book I’m dealing with. When someone requests a story, I don’t just grab any book at random. Instead, I look at the emblem to confidently select the right type of book, ensuring they get exactly what they want.

One day, a young wizard visited my library seeking a book that could teach him spells. Thanks to the discriminated unions, I instantly knew to hand him a textbook with a wand emblem on its spine. This emblem acted as a type check, guaranteeing that I wouldn’t mistakenly hand him a comic or a novel. It was all about precision, just like a spell that requires the exact incantation to work.

This emblem system not only made my job easier but also ensured that the library ran smoothly, avoiding any mishaps. It was like having a built-in type safety net, preventing errors and ensuring everyone got precisely what they needed.

So, in this library, discriminated unions and their emblems became my trusted allies, allowing me to maintain order and ensure that every reader’s experience was just as enchanting as they imagined.

---

Consider this TypeScript example:

type Book = 
  | { kind: 'novel'; title: string; author: string }
  | { kind: 'textbook'; title: string; subject: string }
  | { kind: 'comic'; title: string; illustrator: string };

function describeBook(book: Book): string {
  switch (book.kind) {
    case 'novel':
      return `Novel: "${book.title}" by ${book.author}`;
    case 'textbook':
      return `Textbook: "${book.title}" on ${book.subject}`;
    case 'comic':
      return `Comic: "${book.title}" illustrated by ${book.illustrator}`;
    default:
      // This case should never happen if all possible types are covered
      return 'Unknown book type';
  }
}

In this code, the kind property acts like the emblem on the book’s spine, discriminating between different types of books. When I use the describeBook function, the kind ensures that I handle each book type correctly. TypeScript checks that all possible types are covered in the switch statement, which prevents errors and ensures type safety—just like how I confidently select the right book for each reader.

Key Takeaways:

1. Type Safety: Discriminated unions provide a clear and safe way to handle different data types in TypeScript, akin to identifying books by their emblems.
2. Error Prevention: By using a discriminating property (like kind), we prevent mistakes and ensure that each type is handled appropriately.
3. Code Clarity: This approach makes the code more understandable and maintainable, as each type is clearly defined and managed.