Typescript子类类型（类型断言）

Question

I hope you like animals. Here is a speaking example :

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

The error : Type 'Animal' is not assignable to type 'Cat'. Property 'Miaou' is missing in type 'Animal'.

Somebody can explain me the difference ? I thought there were none...

EDIT : OK, It is detailed in the typescript specification : Type Assertions

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

Answer 1

The "retrieveAnimal" function return an object of the "Animal" type, but here

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

you declare "retrievedCat" variable of the "Cat" type, so you indeed can not cast Animal to Cat.

In the second case:

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

you declare "retrievedCat" variable of the "any" type (you doesn't specify any type, so by default - "any"), and specify value as "Cat". Obviously, you can cast "Cat" to "any", IMHO.