carlrip

function logStatus(status: "LOADING" | "LOADED") {
  console.log(status);
}

const Status = {
  Loading: "LOADING",
  Loaded: "LOADED",
};

logStatus(Status.Loading); // 💥 - Type error - Argument of type 'string' is not assignable to parameter of type '"LOADING" | "LOADED"'

carlrip

type Person = {
  id: number;
  name: string;
  scores: number[];
}
const people: Person[] = [
  { id: 1, name: "Bob", scores: [50, 45] },
  { id: 2, name: "Jane", scores: [70, 60] },
  { id: 3, name: "Paul", scores: [40, 75] }
]

carlrip

function createGetPersonAction() {
  return { type: 'GetPerson' } as const;
}
const getPersonAction = createGetPersonAction();  // `getPersonAction` is of type `{ readonly type: "GetPerson"; }`

carlrip

type Person = {
  readonly name: string;
  readonly scores: ReadonlyArray<number>;   // same as readonly number[]
}

carlrip

type Person = {
  readonly name: string;
  readonly scores: readonly number[];
}
const bob: Person = {
  name: "Bob",
  scores: [50, 45]
}
bob.scores.push(60);  // 💥 - Type error - Property 'push' does not exist on type 'readonly number[]'

carlrip

type Person = {
  readonly name: string;
  readonly scores: number[];
}
const bob: Person = {
  name: "Bob",
  scores: [50, 45]
}

bob.scores.push(60);  // does this raise a type error?

carlrip

const person: unknown = await getPerson(id);
if (isPersonWithAddress(person)) {
  const postcode = person.address.postcode; 
}

function isPersonWithAddress(person: any): person is Person {
  return "address" in person && "postcode" in person.address;
}

carlrip

const person: any = await getPerson(id);
const postcode = person.address.postcode;  // TypeScript doesn't yell at us but what if there is no address property ... 💥!

carlrip

const [state, setState] = useState(initialState);

carlrip

type Reset = {
  type: "reset";
  to: number;
};
type Actions = Increment | Decrement | Reset;