Discriminated unions?

[ivnsch]

I was trying to switch ("match style") over enum cases with fields, but this seems not to be possible. Example (src):

type AppEvent =
  | { kind: "click"; x: number; y: number }
  | { kind: "keypress"; key: string; code: number }
  | { kind: "focus"; element: HTMLElement };

function handleEvent(event: AppEvent) {
  switch (event.kind) {
    case "click":
      // We know it is a mouse click, so we can access `x` and `y` now
      console.log(`Mouse clicked at (${event.x}, ${event.y})`);
      break;
    case "keypress":
      // We know it is a key press, so we can access `key` and `code` now
      console.log(`Key pressed: (key=${event.key}, code=${event.code})`);
      break;
    case "focus":
      // We know it is a focus event, so we can access `element`
      console.log(`Focused element: ${event.element.tagName}`);
      break;
  }
}

Potential feature request, or maybe it's already possible and I'm missing something?

[jkomyno]

Hi @ivanschuetz, it seems you're referring to a TypeScript limitation (which I believe is solved in TypeScript v4.9, btw).
For type-safe pattern matching in TypeScript, I strongly recommend the excellent ts-pattern library.

[stefnotch]

I think this issue can be closed, since it's about Typescript and not about something that this library provides ^

And yes, it works with new versions of Typescript
https://www.typescriptlang.org/play?#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-sYIeFItFAVaEQ4kSi0YCAGJBe4EhsZGJQH0UgcU4AVGwAOQqUSUkYlsfjnhUuCUQA