缩小类型的范围

function padLeft(padding: number | string, input: string): string {
  if (typeof padding === "number") {
    return " ".repeat(padding) + input;
  }
  return padding + input;
}

缩小真假范围

function getUsersOnlineMessage(numUsersOnline: number) {
  if (numUsersOnline) {
    return `There are ${numUsersOnline} online now!`;
  }
  return "Nobody's here. :(";
}

根据in运算符缩小范围

type Fish = { swim: () => void };
type Bird = { fly: () => void };
 
function move(animal: Fish | Bird) {
  if ("swim" in animal) {
    return animal.swim();
  }
 
  return animal.fly();
}

根据instanceof缩小范围

function logValue(x: Date | string) {
  if (x instanceof Date) {
    console.log(x.toUTCString());
  } else {
    console.log(x.toUpperCase());
  }
}

根据控制流自动分析

function example() {
  let x: string | number | boolean;

  x = Math.random() < 0.5;

  console.log(x); // let x: boolean

  if (Math.random() < 0.5) {
    x = "hello";
    console.log(x); // let x: string
  } else {
    x = 100;
    console.log(x); // let x: number
  }

  return x; // let x: string | number
}

通过类型断言

function isFish(pet: Fish | Bird): pet is Fish {
  return (pet as Fish).swim !== undefined;
}

区分联合类型

interface Circle {
  kind: "circle";
  radius: number;
}
 
interface Square {
  kind: "square";
  sideLength: number;
}

 
type Shape = Circle | Square;

function getArea(shape: Shape) {
  switch (shape.kind) {
    case "circle":
      return Math.PI * shape.radius ** 2; // (parameter) shape: Circle
    case "square":
      return shape.sideLength ** 2; // (parameter) shape: Square
    default:
      // never 可以检查是否处理全部情况
      const _exhaustiveCheck: never = shape;
      return _exhaustiveCheck;
  }
}