TypeScript class decorator that modifies object instance

Question

I\'m making a plugin for Aurelia and need a class decorator that

1. adds attributes to the new object instance, and
2. calls an external function with the new

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)