How to parametrize the class decorator function?

Question

I have the following test code, collected from examples in the internet:

function Dec(target: any) {
  console.log("Dec called with: " + JSON.stringify(target));
}

@Dec
class C {
  public x: number = 0;
  constructor() {
    this.x = 5;
  }
  show() {
    console.log("C.show(): " + this.x);
  }
}

let c = new C();
c.show();

Compiling with tsc --experimentalDecorators test.ts I get no warning or any problem. But calling it (with a node test.js command), I get the following result:

Dec called with: undefined
C.show(): 5

I think, the decorators doesn't work on the way, as the most examples (for typescript 1.5) around on the net shows.

How to implement this decorator correctly?

---

The resulting javascript code is this:

var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
function Dec(target) {
    console.log("Dec called with: " + JSON.stringify(target));
}
var C = (function () {
    function C() {
        this.x = 0;
        this.x = 5;
    }
    C.prototype.show = function () {
        console.log("C.show(): " + this.x);
    };
    return C;
}());
C = __decorate([
    Dec
], C);
var c = new C();
c.show();

Answer 1

From the Documentation:

The class decorator is applied to the constructor of the class and can be used to observe, modify, or replace a class definition.

The argument being passed as target is actually the constructor function of your C class (and not the actual instance created). Obviously, trying to JSON.stringify a function reference will yield undefined as a result.