Unreal.js doesn't require any meta data to expose into Javascript because UnrealEngine has already enough information of UCLASS, UPROPERTY and UFUNCTION. So if you want some functions to be used within *.js, you may declare a UBlueprintFunctionLibrary. This doesn't require the whole engine to be rebuilt.

class UMyBlueprintFunctionLibrary : public UBlueprintFunctionLibrary
{
  GENERATED_BODY()
public:
  static void ReallyNiceFunction(ASomeSneakyActor* Actor, int Parameter);
];

let actor = new ASomeSneakyActor(GWorld)
UMyBlueprintFunctionLibrary.ReallyNiceFunction(actor, 1)
actor.ReallyNiceFunction(1)

Boring window extension

A serious example