How to check if Child class has overridden Parent method/function?

Question

I'm mocking an interface class:

const error = "Child must implement method";

class MyInterface
{
  normalFunction()
  {
    throw error;
  }

  async asyncFunction()
  {
    return new Promise(() => Promise.reject(error));
  }
}

class MyImplementation extends MyInterface
{
}

If any of the interface methods are called without an overridden implementation, the error gets thrown. However these errors will only appear at time of execution.

Is there a way to check that the functions were overridden at construction?

"at construction": you mean at construction of an instance of `MyImplementation`? — trincot, May 03 '19 at 14:20
You will have to implement something like a factory pattern to perform that check. I don't think there is something native to JS to do this. — sjahan, May 03 '19 at 14:46
BTW: you can just `throw` the error in the `async` function: it will result in the rejection of the promise that is returned. — trincot, May 03 '19 at 15:09
@trincot Yes. I did that to reinforce the "asynchronicity" of the function. I'm a little busy atm, but will take a look at your solution soon — sookie, May 03 '19 at 15:13

trincot · Accepted Answer · 2019-05-03T15:08:09.203

You could add some inspection in the constructor of MyInterface, like this:

class MyInterface {
    constructor() {
        const proto = Object.getPrototypeOf(this);
        const superProto = MyInterface.prototype;
        const missing = Object.getOwnPropertyNames(superProto).find(name =>
            typeof superProto[name] === "function" && !proto.hasOwnProperty(name)
        );
        if (missing) throw new TypeError(`${this.constructor.name} needs to implement ${missing}`);
    }

    normalFunction() {}

    async asyncFunction() {}
}

class MyImplementation extends MyInterface {}

// Trigger the error:
new MyImplementation();

Note that there are still ways to create an instance of MyImplementation without running a constructor:

Object.create(MyImplementation.prototype)

score 0 · Answer 2 · answered May 03 '19 at 14:49

Can't you use reflection in order to list all the function of a class?

For example here https://stackoverflow.com/a/31055217/10691359 give us a function which list all function of an object. Once you get all of them, you can see if you have or not an overridden function.

How to check if Child class has overridden Parent method/function?

2 Answers2