Typescript子类类型（类型断言）

Question

class Animal {
  constructor(public name: string, public age: number) {}
}

class Cat extends Animal {
  constructor(name: string, age: number) {
    super(name, age);
  }
  public miaou() {
    console.log('Miaou');
  }
}

class Kennel {
  animals = Map<string, Animal> new Map();

  public addAnimal(animal: Animal): void {
    this.animals.set(animal.name, animal);
  }

  public retrieveAnimal(name: string): Animal {
    return this.animals.get(name);
  }
}

let kennel = <Kennel> new Kennel();
let hubert = <Cat> new Cat('Hubert', 4);

kennel.addAnimal(hubert);

let retrievedCat: Cat = kennel.retrieveAnimal('Hubert'); // error  
let retrievedCat = <Cat> kennel.retrieveAnimal('Hubert'); // Works

编辑：OK，在typescript规范中有详细说明：类型断言

class Shape { ... }
class Circle extends Shape { ... }
function createShape(kind: string): Shape {
 if (kind === "circle") return new Circle();
 ...
}
var circle = <Circle> createShape("circle");

Answer 1

RetrieveAnimal“函数返回一个”Animal“类型对象，但在这里

let retrievedCat: Cat = kennel.retrieveAnimal('Hubert');

您声明了“cat”类型的“retrievedcat”变量，因此您确实不能将Animal转换为cat。

在第二种情况下：

let retrievedCat = <Cat> kennel.retrieveAnimal('Hubert');