传递通用类类型作为通用函数的参数

Question

I'm using a converter (external library) that has a method fromJson like the one below. I want to build a generic RESTService, but i don't know how to pass the generic class type to the method.

// From the library
export declare type Clazz<T> = {
  new (...args: any[]): T;
};

// From the library
export class Converter {
  ...
  public fromJson<T>(json: any, clazz: Clazz<T>): T { ... }
  ...
}

// ******************************************************

// My Service on my Angular app
class RESTService<T> {
  ...
  public save(t: T): Observable<T> {
    return this.httpClient.post('http://...', t)
      .pipe(map(res => this.converter.fromJson(res, ???)));
  }
  ...
}

Answer 1

You can use t.constructor as the Clazz<T> argument. You'll need to tell TypeScript it is of the right type using as :

// From the library
declare type Clazz <T> = {
    new(...args: any[]): T;
};

// From the library
class Converter {
    public fromJson<T>(json: any, clazz: Clazz<T>) { return new clazz(); }
}

class Test {
    public testIt<T>(test: T) {
        const converter = new Converter();
        var o = converter.fromJson("{}", test.constructor as new (...args: any[]) => T);
        return o;
    }
}

class TestObj {
    testProp: string = "hello";
}

var o = new TestObj();
var t = new Test();
var a = t.testIt(o);
console.log(JSON.stringify(a));

This seems to compile and work in the TypeScript Playground , but as always, you should test it out in your own environment.

I did alter fromJson just so that I could get a loggable result, and obviously testIt is not exactly the same as your (Angular?) function, but hopefully it gets you going in the right direction.

Answer 2

1. The library you are using is very bad and has semantic errors. new is a keyword used to initialize an instance of a class type. The return type is should always be consistent with this class type. The correct definition should be:

    export declare type Clazz<T> = { new (...args: any[]): Clazz<T>; }; 

2. Solve this problem. You can always use the keyword as and Object.create(null) to convert most object types to avoid compiler checking:

    class RESTService<T> { ... public save(t: T): Observable<T> { let clazz = Object.create(null) as Clazz<T>; // clazz.someValue = 'Hello world'; return this.httpClient.post('http://...', t) .pipe(map(res => this.converter.fromJson(res, clazz))); } ... } 

   Object.create(null) give you a js object without any method binding --- a pure struct space.