TypeScript class decorator that modifies object instance - class

Question

I'm making a plugin for Aurelia and need a class decorator that
adds attributes to the new object instance, and
calls an external function with the new object as an argument.
I've looked through examples, and so far I've put together ("pseudo-ish" code)
return function addAndCall(target: any): any {
var original = target;
var newConstructor = function (...args) {
original.apply(this, args);
this.newAttribute = "object instance value";
ExternalModule.externalFunction(this);
};
newConstructor.prototype = Object.create(original.prototype);
newConstructor.prototype.constructor = original;
return <any>newConstructor;
}
but
I'm not entirely clear on the details here (or what is actually needed), and
it might not work properly since I'm getting Aurelia errors when using objects instantiated from classes with this decorator (and I suspect it's my decorator rather than the Aurelia framework that's buggy).
Any help and explanation would be greatly appreciated!

Answer 1

Why not just assign those properties to the prototype, and subsequently assign to the instance on first invocation
// decorator
function addAndCall(cb: Function, newField: string) {
// cb is now available in the decorator
return function(ctor: Function): void {
Object.defineProperty(ctor.prototype, newField, {
value: function(...args: any[]) {
return Object.defineProperty(this, newField, {
value: function(...args: any[]) {
console.log(newField, ...args);
}
})[newField](...args);
}
});
cb(ctor);
}
}
let callMe = (decoratedCtor) => console.log(decoratedCtor);
#addAndCall(callMe, 'propertyName')
class AddToMe {}
let addToMe = new AddToMe();
(<any>addToMe).propertyName(1, 2);

Answer 2

Here's a working version:
function addAndCall(target: any) {
var original = target;
function construct(constructor, args) {
var c: any = function () {
this.newAttribute = "object instance value";
ExternalModule.externalFunction(this);
return constructor.apply(this, args);;
}
c.prototype = constructor.prototype;
return new c();
}
var f: any = function (...args) {
return construct(original, args);
}
f.prototype = original.prototype;
return f;
}
(code in playground)