How to create ActionFilter in Javascript?

I understand that there is a concept of Action Filters in languages like C# using ASP.NET. I would like to have this kind of pre-processing functionality in my class methods, using Javascript. Is this possible/a part of the language? Otherwise how would I implement this feature? It sounds like a language level implementation, like constructors and getters, and so I’m confused whether or not it’s possible to implement such a thing.

TMI: I’m trying to create an ecommerce website with firebase auth. Right now I have implemented all the cart methods; but I would like to have some sort of auth check to allow these methods to be called, or otherwise route the user to the login page.

Answer

While I couldn’t find the exact functionality of an Action Filter, JS has a neat feature called Proxy‘s, which can be used for the same purpose. When you create a proxy of an object, it intercept the requests to the given object, and sort of acts like an intermediary. What I found is that you can include the processing logic of an Action Filter, such as checking if a user is authorized, inside the get and set of the handler provided to the proxy.

    const baseInstance = new SomeClass(param1, param2);
    const handler = {
      get(target, prop) {
        if(/* implement required check */) alternatePath();
        else return target[prop];
      },
    };

    const extendedInstance = new Proxy(baseInstance, handler);

This eliminates the need to modify each method of the base class, just to include that one check(or multiple checks; that can also be done). Doing so, we separate the functionalities. Check out this neat video for more info.