Calling abstract methods inside abstract class constructor

[prashaantt]

TypeScript Version:

1.8.10

Code

export abstract class AbstractClass {
    constructor() {
        this.abstractMethod();
    }

    abstract abstractMethod(): void;
}

Output

"use strict";
var AbstractClass = (function () {
    function AbstractClass() {
        this.abstractMethod();
    }
    return AbstractClass;
}());
exports.AbstractClass = AbstractClass;

Question

Is there a situation where calling this.abstractMethod inside the constructor makes sense? I have accidentally hit this pattern and the class compiles successfully, but the resulting JavaScript obviously fails because this.abstractMethod is not a function.

[blendsdk]

How about a base class where it needs to configure itself based on the actual implementation of de derived/consumer class.

[prashaantt]

@blendsdk Yes, I was trying something similar, but the compiled JS fails to run so I don't think I got that right. Besides, the output looks buggy to me.

[blendsdk]

Here is an example: https://github.com/blendsdk/trunk/tree/master/abstract

>> tsc
>> node dist/ConsumerClass.js
>> Hey

[mhegazy]

the methods would work. properties would not. not sure if there is much we can do here though.

[aluanhaddad]

How about a base class where it needs to configure itself based on the actual implementation of de derived/consumer class.

Isn't this dangerous? Isn't super being called before the derived class is fully initialized?

[mhegazy]

methods go on the prototype. so does not matter when you call the method, it should be there. properties are added to the instance after super is called in the constructor. so they will not be set.

the issue is we can prohibit the use of abstract properties in the constructor, and we probably should, but you can call a method from the constructor and the method tries to use a property, this we can not check.

[aluanhaddad]

the issue is we can prohibit the use of abstract properties in the constructor, and we probably should, but you can call a method from the constructor and the method tries to use a property, this we can not check.

That is exactly what I meant. I was sort of saying that calling a derived method from a base class constructor is probably a bad idea. It is certainly discouraged in many languages. Perhaps it should be an error if a method marked as abstract is called from a constructor implementation within the same class.

Edit: I'm not sure it matters that the base class is marked abstract, just that the method is.

[prashaantt]

FWIW, I ran into this trying to make ConcreteClass have a specified constructor shape and staticMethod:

// abstract.ts

export abstract class AbstractClass {
    constructor(s: string) {
        this.abstractMethod(parseInt(s));
    }

    abstract abstractMethod(n: number): void;
}

// interface.ts

export interface Interface {
    new (s: string): AbstractClass;
    staticMethod(): void;
}

// concrete.ts

export class ConcreteClass extends AbstractClass {
    abstractMethod(n: number) { }
    static staticMethod() { }
}

export const concreteClass: Interface = ConcreteClass;

This works great in TS, but then emits that JS above. ConcreteClass works fine but AbstractClass fails to instantiate on its own.

I believe this is a valid requirement, but is there another way to achieve this?

[prashaantt]

By the way, I'm still new to TypeScript so the other thing I'm curious about here is, why can't the Language Service not automatically infer that my derived class (not declared abstract) needs to implement abstractMethod and offer me the IntelliSense for it with auto-complete? And why should I need to declare the argument types for abstractMethod again when they can also be automatically inferred from the declaration in the base class?

[blendsdk]

To my best knowledge these are handled by the IDE. If you use VSCode, it will complain about the abstract method not being implemented in the derived class. At the moment VSCode does not auto-generate code for the abstract function.

[prashaantt]

it will complain about the abstract method not being implemented in the derived class

Yes, that it does. But perhaps it could go a step further in suggesting the method name and signature as well inside the derived class block.

[blendsdk]

Yes agreed. It is best to address these issues at VSCode. The typescript team cannot do much about it here I guess.

[aluanhaddad]

To my best knowledge these are handled by the IDE. If you use VSCode, it will complain about the abstract method not being implemented in the derived class. At the moment VSCode does not auto-generate code for the abstract function.

VSCode does not do any code generation for TypeScript. It is all generated via the TypeScript compiler.

Furthermore, the TypeScript language service itself is responsible for producing errors in VSCode. Ide plugins like tslint can produce warnings. Neither effect code generation.

If you're using WebStorm, or Resharper in Visual Studio, JetBrains has their own language service but I would be shocked if the actual generated code was different.