Typescript Compiler Does Not Know About ES6 Proxy Trap on Class
As an addition to the accepted answer I want to share a solution that can be used for more simple cases where you simply want to be able to proxy to any method or property (either a trap in the handler or something that actually exists on the actual target). The solution is based on using an "index signature", read more on that topic here: https://www.typescriptlang.org/docs/handbook/interfaces.html
I hope it will be useful for others ending up at this great question:
const handler = {
get: (target: object, property: string, receiver: any) => {
// define custom traps here:
if (property === "trap") {
// ... do something ...
}
// optional test whether property exists using the Reflect class:
if (!Reflect.has(target, property)) {
throw Error(`Property ${property} does not exist`);
}
// proxy to the target using the Reflect class:
return Reflect.get(target, property);
},
};
I will show two solutions:
1. create a factory and an interface:
// Any property is allowed in this interface using the following index signature:
interface FooInterface {
[key: string]: any;
}
// From the factory we return the FooInterface
const proxyFactory = (target: Foo): FooInterface => {
return new Proxy(target, handler);
};
2. create a class with an index signature and return the proxy directly from the class constructor
class FooProxy {
[key: string]: any;
constructor(target: Foo) {
return new Proxy(target, handler);
}
}
Can be used like this where all possible methods and properties you want to build a trap for inside the handler will be recognized:
const fooProxy = proxyFactory(foo);
or
const fooProxy = new FooProxy(foo);
No more messages complaining:
Property '...property name...' does not exist on type 'Proxy'.
So anything can be called and trapped now:
const returnedFromMethod = fooProxy.anyMethod();
const property = fooProxy.anyProperty;
You can of course refine your interface, the example is simply to demonstrate a solution for all methods and properties.
I don't think in this case a class is the best approach you can just use a function to create the proxy and all will work as expected:
function createFooProxy(foo:Foo) : Foo { // Proxy<Foo> is compatible with Foo
let handler = {
get: function(target: Foo, prop: keyof Foo, receiver: any) {
if(Foo.prototype[prop] !== null) {
return foo[prop];
}
return Reflect.get(target, prop, receiver);
}
}
return new Proxy(foo, handler);
}
If you are set on using the class approach, you can fake a base class:
function fakeBaseClass<T>() : new() => Pick<T, keyof T>{ // we use a pick to remove the abstract modifier
return class {} as any
}
class FooProxy extends fakeBaseClass<Foo>(){
private foo: Foo; // I would make this private as it is not really accessible on what the constructor of FooProxy returns (maybe remove it as I see no use for it)
constructor(foo: Foo) {
super();
this.foo = foo;
let handler = {
get: function(target: FooProxy, prop: keyof Foo, receiver: any) {
if(Foo.prototype[prop] !== null) {
return target.foo[prop];
}
return Reflect.get(target, prop, receiver);
}
}
return new Proxy(this, handler);
}
}