in, instanceof และยูเนียนแบบจำแนก

type HasLength = { length: number };

function print(v: string | HasLength) {
  if ("length" in v) {
    // v is HasLength
    console.log("len:", v.length);
  } else {
    // v is string
    console.log(v.toUpperCase());
  }
}

print({ length: 3 });
print("ts");

class Person {
  constructor(public name: string) {}
}

function hello(x: Person | Date) {
  if (x instanceof Person) {
    console.log("Hi", x.name);
  } else {
    console.log("Year", x.getFullYear());
  }
}

hello(new Person("Ada"));
hello(new Date(2024, 0, 1));

type Circle = { kind: "circle"; radius: number };
type Square = { kind: "square"; size: number };

type Shape = Circle | Square;

function area(s: Shape): number {
  switch (s.kind) {
    case "circle":
      return Math.PI * s.radius ** 2;
    case "square":
      return s.size * s.size;
  }
}

console.log(area({ kind: "circle", radius: 2 }));
console.log(area({ kind: "square", size: 3 }));

type FetchOk = { status: "ok"; data: { length: number } };
type FetchErr = { status: "error"; message: string };

type Result = FetchOk | FetchErr;

function handle(r: Result) {
  if (r.status === "ok" && "length" in r.data) {
    console.log("items:", r.data.length);
  } else {
    console.log("error:", r.message);
  }
}

handle({ status: "ok", data: { length: 5 } });
handle({ status: "error", message: "oops" });