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15.3: Exhaustiveness Checks With Never - Coggle Diagram

15.3: Exhaustiveness Checks With Never

example

function getSpeed(animal: Animal) {
  switch (animal.type) {
    case 'bird':
      return animal.flyingSpeed;
    case 'horse':
      return animal.runningSpeed;
    case 'cow':
      return animal.walkingSpeed;
    default:
      return 0;
      // We should never make it here!
  }
}

interface Bird {
  type: 'bird';
  flyingSpeed: number;
}
interface Horse {
  type: 'horse';
  runningSpeed: number;
} 
interface Cow {
  type: 'cow';
  walkingSpeed: number;
}

type Animal = Bird | Horse | Cow;

by using never

used as a return type for a function

function generateError(message: string, code: number): never {
  throw { message: message, errorCode: code }
}

add a new Animal

common pattern used

TypeScript will complain

> Type 'Rabbit' is not assignable to type 'never'

function getSpeed(animal: Animal) {
  switch (animal.type) {
    case 'bird':
    ...
    default:
      // We should never make it here!
      const unhandledType: never = animal;
      return unhandledType;
  }
}

but if we add the type in

function getSpeed(animal: Animal) {
  switch (animal.type) {
    case 'bird':
    ...
    case 'rabbit':
      return animal.hoppingSpeed;
    default:
      // We should never make it here!
      const unhandledType: never = animal;
      return unhandledType;
  }
}

type Animal = Bird | Horse | Cow | Rabbit;