How to employ Proxy<T> with a type other than T as the parameter?

I find myself in a situation where I am interested in utilizing the Proxy feature to implement "load balancing" among a collection of classes.

To illustrate my approach, consider the following simple example:

class Foo {
    constructor(private msg: string) {}

    foo() {
        console.log(this.msg);
    }
}

// @ts-ignore
const proxy: Foo = new Proxy([new Foo('foo'), new Foo('bar')], {
    get: (o, key) => {
        const client = o[Math.floor(Math.random() * o.length)];
        console.log(client, key);
        return client[key];
    },
});
proxy.foo();

This implementation seems to be functional. However, things become complicated when dealing with TypeScript. The type definition of Proxy hinders us from carrying out something like:

new Proxy<Foo>([new Foo(), new Foo()], handler)

This particular code snippet leads to an error message stating:

Argument of type 'Foo[]' is not assignable to parameter of type 'Foo'.

Is there any way to accomplish this task without sacrificing type safety?

Answer №1

Just enhancing the current definitions is sufficient; no need for a complete overhaul.

In case you are implementing a module structure, it is essential to redefine the ProxyConstructor globally for proper functionality:

declare global  {
    interface ProxyConstructor {
        new <TSource extends object, TTarget extends object>(target: TSource, handler: ProxyHandler<TSource>): TTarget;
    }
}


const proxy: Foo = new Proxy<Foo[], Foo>([new Foo('foo'), new Foo('bar')], {
    get: (o, key) => {
        const client = o[Math.floor(Math.random() * o.length)];
        console.log(client, key);
        return client[key];
    },
});
proxy.foo();

Answer №2

An efficient approach is to implement a factory function such as the one below:

function distributeBalance<T>(items: Array<T>): T {
  return new Proxy<any>({}, {
    get: (object, key) => {
        const item = items[Math.floor(Math.random() * items.length)];
        console.log(item, key);
        return item[key];
    },
  }) as T;
}

const balancedProxy = distributeBalance([new Widget('blue'), new Widget('red')]);
balancedProxy.color();

This method allows you to create a reusable and type-safe balancer without any compromise on declaration.

Answer №3

To enhance the type definition of Proxy, you can make it possible for a different type to be used instead of its parameter type.

interface ProxyConstructor {
    revocable<T extends object, S extends object>(
        target: T,
        handler: ProxyHandler<S>,
    ): { proxy: T; revoke: () => void };
    new <T extends object>(target: T, handler: ProxyHandler<T>): T;
    new <T extends object, S extends object>(target: S, handler: ProxyHandler<S>): T;
}
declare var Proxy: ProxyConstructor;

Next, adjust how you use the Proxy as shown below:

const proxy: Foo = new Proxy<Foo, Foo[]>([new Foo('foo'), new Foo('bar')], {
    get: (o, key) => {
        const client = o[Math.floor(Math.random() * o.length)];
        console.log(client, key);
        return client[key];
    },
});

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