Proxies: Invoking static methods from the parent of the target object through a proxy

Question

Proxies: Invoking static methods from the parent of the target object through a proxy

Here's an intriguing challenge for JavaScript enthusiasts

Can you intercept property access in JavaScript using proxies?

By applying a clever hack like the one demonstrated below, it's possible to intercept static property access within a class:

class Handler{
    constructor(object){
        this.object = object;
    }

    get(target, property){
        if (property === 'all') {
            return () => `selecting data from table: ${this.object.table}` ;
        }
        return target[property];
    }
}
class User{
    static table = 'users'
}

Object.setPrototypeOf(User, new Proxy({}, new Handler(User)));

console.log(User.all()); // prints "selecting data from table: users"

The issue arises when attempting to extend the User class and then invoking a static method from the parent class of User:

class Handler{
    constructor(object){
        this.object = object;
    }

    get(target, property){
        if (property === 'all') {
            return () => `selecting data from table: ${this.object.getTable()}` ;
        }
        return target[property];
    }
}

class Model{
    static getTable(){return this.table;}
}

class User extends Model{
    static table = 'users'
}

Object.setPrototypeOf(User, new Proxy({}, new Handler(User)));

console.log(User.all());

Executing this code will result in

TypeError: this.object.getTable is not a function

.

Upon closer inspection, I discovered that this.object is not a User class but rather some kind of a function.

Is there a way, through any workaround, to invoke the parent's static method getTable?

The second issue, even if you resolve the first one, is that instantiating the User class becomes impossible:

console.log(new User());
TypeError: Super constructor [object Object] of User is not a constructor

I suspect this is because User no longer functions as a class: console.log(User) yields ƒ [object Function] instead of class User extends Model

Does JavaScript support this type of functionality?

javascript inheritance static prototype es6-proxy

Answer 1

Answer №1

When working with your code, it becomes apparent that calling getTable() from the Model class within the User class is not possible anymore due to changes in their parent-child relationship.

To better understand this change, let's examine the prototype chain after running the provided code:

class Model{
    static getTable(){return this.table;}
}

class User extends Model{
    static table = 'users'
}

console.log(Object.getPrototypeOf(Model));
// Function (as class is a constructor function in JS)
// Model.[[Prototype]] === Function.prototype

console.log(Object.getPrototypeOf(User));
// Class Model with getTable() method
// User.[[Prototype]] === Model.prototype

Initially, the prototype chain follows the expected structure. However, upon executing

Object.setPrototypeOf(User, new Proxy({}, new Handler(User)));

, the inheritance is altered resulting in the following hierarchy:

const userPrototype = Object.getPrototypeOf(User);
console.log(userPrototype);
// Proxy {} (User now inherits from a Proxy instance)

const userPrototypePrototype = Object.getPrototypeOf(userPrototype);
console.log(userPrototypePrototype);
// Object instance methods (due to Proxy target being {})

Object.getPrototypeOf(userPrototypePrototype);
// null (end of prototype chain)

This modification removes Model from the chain, preventing direct access to its methods like getTable() from within User.

To achieve desired results without directly altering the prototype chain, an alternative solution is proposed:

class Handler{
    constructor(object){
        this.object = object;
    }
    
    get(target, property){
        if (property === 'all') {
            return () => `selecting data from table: ${this.object.getTable()}` ;
        }
        return target[property];
    }
}

class Model{
    static getTable(){return this.table;}
}

class User extends Model{
    static table = 'users'
}

class ProxyClass {
    constructor(object) {
        return new Proxy(object, new Handler(object));
    }
}

const UserProxy = new ProxyClass(User);

console.log(UserProxy.all());
// selecting data from table: users

console.log(new UserProxy());
// User {}

This approach allows dynamic wrapping of objects in a proxy while retaining access to inherited methods and class instantiation capabilities.

Important reminder! As noted on MDN page Inheritance with the prototype chain:

The someObject.[[Prototype]] notation refers to the prototype of someObject as per ECMAScript standard ac-cessed via Object.getPrototypeOf() and Object.setPrototypeOf(). This differs from the non-standard __proto__ property used by some browsers.

It should not be confused with func.prototype which assigns the [[Prototype]] to instances created by a function. The Object.prototype represents the base Object prototype.

Answer 2

When working with your code, it becomes apparent that calling getTable() from the Model class within the User class is not possible anymore due to changes in their parent-child relationship.

To better understand this change, let's examine the prototype chain after running the provided code:

class Model{
    static getTable(){return this.table;}
}

class User extends Model{
    static table = 'users'
}

console.log(Object.getPrototypeOf(Model));
// Function (as class is a constructor function in JS)
// Model.[[Prototype]] === Function.prototype

console.log(Object.getPrototypeOf(User));
// Class Model with getTable() method
// User.[[Prototype]] === Model.prototype

Initially, the prototype chain follows the expected structure. However, upon executing

Object.setPrototypeOf(User, new Proxy({}, new Handler(User)));

, the inheritance is altered resulting in the following hierarchy:

const userPrototype = Object.getPrototypeOf(User);
console.log(userPrototype);
// Proxy {} (User now inherits from a Proxy instance)

const userPrototypePrototype = Object.getPrototypeOf(userPrototype);
console.log(userPrototypePrototype);
// Object instance methods (due to Proxy target being {})

Object.getPrototypeOf(userPrototypePrototype);
// null (end of prototype chain)

This modification removes Model from the chain, preventing direct access to its methods like getTable() from within User.

To achieve desired results without directly altering the prototype chain, an alternative solution is proposed:

class Handler{
    constructor(object){
        this.object = object;
    }
    
    get(target, property){
        if (property === 'all') {
            return () => `selecting data from table: ${this.object.getTable()}` ;
        }
        return target[property];
    }
}

class Model{
    static getTable(){return this.table;}
}

class User extends Model{
    static table = 'users'
}

class ProxyClass {
    constructor(object) {
        return new Proxy(object, new Handler(object));
    }
}

const UserProxy = new ProxyClass(User);

console.log(UserProxy.all());
// selecting data from table: users

console.log(new UserProxy());
// User {}

This approach allows dynamic wrapping of objects in a proxy while retaining access to inherited methods and class instantiation capabilities.

Important reminder! As noted on MDN page Inheritance with the prototype chain:

The someObject.[[Prototype]] notation refers to the prototype of someObject as per ECMAScript standard ac-cessed via Object.getPrototypeOf() and Object.setPrototypeOf(). This differs from the non-standard __proto__ property used by some browsers.

It should not be confused with func.prototype which assigns the [[Prototype]] to instances created by a function. The Object.prototype represents the base Object prototype.

Proxies: Invoking static methods from the parent of the target object through a proxy

Answer №1

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