如何推断在Typescript方法中绑定到参数化类的通用参数的通用返回类型？

Question

I have the following superclass, T is supposed to be the type the API returns

export class Command<T> {

}

This is a login command that extends the command:

export class LoginCommand extends Command<LoginResult> {
    username: string;
    password: string;
}

And the return object:

export class LoginResult {
    success: boolean;
    token: string;
}

When calling the method:

public call<R>(command: model.command.Command<R>): R {
    return null as R; // code omitted
}

with the following parameter:

const cmd = new LoginCommand();
const success = this.call(cmd).success;

it yields an error : [ts] Property 'success' does not exist on type '{}'

Question 1 : How to modify the method signature to correctly infer the R from Command as the return type? I've also tried the following syntax with the same result:

    public call<T extends Command<R>, R>(command: T): R

Question 2 : Why is the ask method accepting parameters that do not extend Command? Passing in a string yields no errors.

Answer 1

The last question is the easiest to answer, your Command base class has no properties or methods so any type will be structurally equivalent to it including a string.

The other part of the problem is more difficult, if you pass a derived type from a generic type the compiler will not drill down to infer generic parameters.

You could do one of the following:

Add a with operation to the Command class

export class Command<T> {
    private something: "";
    with(fn: (cmd: Command<T>) => T) : T{
        return fn(this);
    }
}

//Usage:
public call<R>(command: Command<R>): R {
    return null as R; // code omitted
}

public doStuff() {
    const cmd = new LoginCommand();
    const success = cmd.with(this.call).success; // Works
}

Add a simple casting method to the base class

export class Command<T> {
    private something: "";
    asCmd(): Command<T> { return this;}
}

public doStuff() {
    const cmd = new LoginCommand();
    const success = this.call(cmd.asCmd()).success;
}