I keep encountering errors with TypeGuard

Looking for some guidance with typescript. Even after implementing a type guard (and including the '?' symbol), I'm still encountering errors in the code snippet below. Is the syntax correct or should I make changes to the tsconfig file?

interface Bird {
  type: "bird";
  speed: number;
  flyingSpeed?: number;
}
interface Snail {
  type: "snail";
  speed: number;
  crawlingSpeed?: number;
}

function accelerateAnimal(animal: Snail | Bird): void {
  switch (animal.type) {
    case "bird":
      speed = speed + animal.flyingSpeed;
      break;
    case "snail":
      speed = speed + animal.crawlingSpeed;
      break;

    default:
      break;
  }
}
accelerateAnimal({type: "bird", speed: 0, flyingSpeed: 3});

Answer №1

flyingVelocity?: number; denotes the flyingVelocity attribute as being optional, allowing it to be set to undefined. TypeScript prohibits the addition of undefined to a number, therefore the statement

speed = speed + bird.flyingVelocity;
will result in an error.

You can either remove the optionality of the property or address the scenario where it is undefined explicitly. To handle the undefined case, you can utilize nullish coalescing to interpret undefined as 0:

speed += bird.flyingVelocity ?? 0;
// the code provided does not include the variable 'speed'
// consider using bird.speed to update the property within the object that is passed in

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