No NAPI access to Javascript Map objects

[nadiasvertex]

• Node.js Version: All
• OS: All
• Scope: NAPI

I am building a C++ plugin for nodejs, and we would like to be able to return a Javascript Map() object to the user. The version of nodejs that we are using clearly supports Map(), but I have been unable to find any support for creating, iterating, or indexing Map objects.

The documentation at https://nodejs.org/api/n-api.html contains no mentions of Map objects at all. The word "map" only appears once on this page and is not used in any context that applies to the collection.

[addaleax]

Yes – at this moment, N-API does not provide any wrapper for Map or Set. You can still use them like regular JS objects though, i.e. use the Map constructor to create objects from N-API and call .set() for adding entries to it.

[nadiasvertex]

I understand. It's awkward and less efficient to have JS compile code fragments every time I need to do something like this, though. Do you have any idea when this will be available? Is there a design spec that just needs implementing?

[addaleax]

@nadiasvertex I guess you could open a ticket in https://github.com/nodejs/node/issues, if you’d like to see this implemented (or are interested in implementing it yourself).

That being said, an approach that I think might be a bit nicer and also be worth exploring would be to create C++ wrappers for Map and Set that use the existing set of N-API primitives, and cache methods like set/delete so they only need to be retrieved once (note that either way, compiling JS code is not required here at all). That could even be provided as a header-only library on npm, with one big advantage being that it would also work across all existing supported Node.js versions.

[nadiasvertex]

I wouldn't be against contributing something. I'd have to clear it with my manager and see where our priorities are.

That being said, the C++ solution you mentioned is very interesting. However I don't see how it would work. First of all, the items in the Map() are not exposed as properties. (I've already tried that.) So I don't know how I would iterate (get_all_property_names doesn't work, I've tried that.)

I assume I'd use napi_new_instance() to create a new map, but I don't see how I would get the constructor for the Map. If you have some pointers, I would appreciate it. Thanks for your time!

[addaleax]

Here’s an example addon that creates a wrapper for Map, sets an entry in the Map, then lists all entries and returns the first one. It’s obviously not very complete or pretty, but I think the idea might become clear. Things like the methods could also be cached in the constructor here.

#include <napi.h>

class NapiMap : public Napi::Object {
 public:
  NapiMap(napi_env env, napi_value value) : Object(env, value) {}

  static NapiMap Create(Napi::Env env) {
    return env.Global().Get("Map").As<Napi::Function>().New({}).As<NapiMap>();
  }

  std::vector<Napi::Array> Entries() {
    std::vector<Napi::Array> result;
    Napi::Object iterator = Get("entries").As<Napi::Function>().Call(*this, {}).As<Napi::Object>();
    Napi::Function next = iterator.Get("next").As<Napi::Function>();
    while (true) {
      Napi::Object entry = next.Call(iterator, {}).As<Napi::Object>();
      if (entry.Get("done").ToBoolean()) break;
      result.push_back(entry.Get("value").As<Napi::Array>());
    }
    return result;
  }

  void Set(Napi::Value key, Napi::Value value) {
    Get("set").As<Napi::Function>().Call(*this, {key, value});
  }
};

Napi::Value Method(const Napi::CallbackInfo& info) {
  Napi::Env env = info.Env();
  NapiMap map = NapiMap::Create(env);
  map.Set(Napi::Number::New(env, 42), Napi::String::New(env, "foo"));
  return map.Entries()[0];
}

Napi::Object Init(Napi::Env env, Napi::Object exports) {
  exports.Set(Napi::String::New(env, "hello"),
              Napi::Function::New(env, Method));
  return exports;
}

NODE_API_MODULE(hello, Init)

[nadiasvertex]

That's very helpful, thanks!

[RaisinTen]

@addaleax is there a similar way of getting a handle to the set of keys of an object? I'm trying to implement an iterator for Napi::Object at nodejs/node-addon-api#930 but currently, I'm keeping a full Napi::Array of keys inside the iterator, which feels incorrect. 😕

[addaleax]

@RaisinTen I don’t think there’s any way to do that, really, no. You can only get a list of all keys at once, unless you use things like for (... in ...).