是否 <ClassName> 在打字稿中执行转换？

Question

The angular DI documentation has the following example:

let mockService = <HeroService> {getHeroes: () => expectedHeroes }

Is mockService now an instance of HeroService ?

Here is another code snippet in case that helps make it clearer:

it('should have heroes when HeroListComponent created', () => {
  let hlc = new HeroListComponent(mockService);
  expect(hlc.heroes.length).toEqual(expectedHeroes.length);
});

I think mockService has to be an instance of the HeroService implementation or interface, because the HeroListComponent expects such an instance it the constructor.

Answer 1

No, it is not an instance of HeroService .
Let's see a simple example:

class Point {
    x: number;
    y: number;

    toString() {
        return `(${this.x}, ${this.y})`;
    }
}

let a = <Point>{ x: 0, y: 0 };

Here a isn't an instance of Point , it just has the same members, but if you'll do console.log(a.toString) you'll get [object Object] instead of (0, 0) .

You can read more about this here: Type assertions , and also you might want to check out this: Type Compatibility .