You're looking for a discriminated union [1], which idiomatically:

  type Dog = { bark(): void; type: 'dog' }

  type Cat = { meow(): void; type: 'cat' }

  function speak(animal: Dog | Cat) {
    if (animal.type === 'dog') {
      animal.bark()
    } else {
      animal.meow()
    }
  }

TypeScript checks your use of JavaScript runtime features.

[1] https://www.convex.dev/typescript/advanced/type-operators-ma...