How to properly wrap constructors with decorators in TypeScript

Question

The process of wrapping a class with a decorator causes superclasses to be unable to access that classes\' properties. Why?

I have some code that:

1. Crea

Answer 1

The comments in the other answers complain that code doesn't work.
Actually, it works, but not in jsFiddle...
It is an issue with the code generation in jsFiddle (perhaps using an obsolete version of TypeScript).
The code above works with TypeScript 2.7.2 (run with Node).

So this is basically the code in pablorsk's answer (except there is no need to return the instance), I just added full types to please a stricter TSLint...

function logClass(): any {
    type Ctor = new (...args: any[]) => T;
    return (target: T): Ctor => {
        // Save a reference to the original constructor
        const Original = target;

        // the new constructor behaviour
        let decoratedConstructor: any = function (...args: any[]): void {
            console.log("Before construction:", Original);
            Original.apply(this, args);
            console.log("After construction");
        };

        // Copy prototype so intanceof operator still works
        decoratedConstructor.prototype = Original.prototype;
        // Copy static members too
        Object.keys(Original).forEach((name: string) => { decoratedConstructor[name] = (Original)[name]; });

        // Return new constructor (will override original)
        return decoratedConstructor;
    };
}

@logClass()
class Base {
    prop = 5;
    constructor(value: number) {
        console.log("Base constructor", value);
        this.prop *= value;
    }
    foo() { console.log("Foo", this.prop); }
    static s() { console.log("Static s"); }
}

class Extended extends Base {
    constructor(init: number) {
        super(init);
        console.log("Extended constructor", init);
    }
    bar() { console.log("Bar", this.prop); }
}

const b = new Base(2);
console.log("Base", b instanceof Base);
b.foo();
Base.s();

const e = new Extended(5);
console.log("Extended", e instanceof Base, e instanceof Extended);
e.bar();

[EDIT] Also added a line copying static members, otherwise decorated class throws an error when calling the static method.