util.ts

/**
 * A decorator that binds values to their class instance.
 * @example
 * class C {
 *   @bind
 *   foo () {
 *     return this;
 *   }
 * }
 * let f = new C().foo;
 * f() instanceof C;    // true
 */
export function bind(target: any, propertyKey: string, descriptor: PropertyDescriptor) {
  return {
    // the first time the prototype property is accessed for an instance,
    // define an instance property pointing to the bound function.
    // This effectively "caches" the bound prototype method as an instance property.
    get() {
      let bound = descriptor.value.bind(this);
      Object.defineProperty(this, propertyKey, {
        value: bound
      });
      return bound;
    }
  };
}