lib/reflect.js
function(trapName) {
    var trap = this.handler[trapName];
    if (trap === undefined) {
      // the trap was not defined,
      // perform the default forwarding behavior
      return undefined;
    }

    if (typeof trap !== "function") {
      throw new TypeError(trapName + " trap is not callable: "+trap);
    }

    return trap;
  }
function(trapName) {
    var trap = this.handler[trapName];
    if (trap === undefined) {
      // the trap was not defined,
      // perform the default forwarding behavior
      return undefined;
    }

    if (typeof trap !== "function") {
      throw new TypeError(trapName + " trap is not callable: "+trap);
    }

    return trap;
  }
If getTrap returns undefined, the caller should perform the
default forwarding behavior.
If getTrap returns normally otherwise, the return value
will be a callable trap function. When calling the trap function,
the caller is responsible for binding its |this| to |this.handler|.