No NAPI access to Javascript Map objects #3303

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1. addaleax commented on Apr 6, 2021

   

   addaleax

   on Apr 6, 2021

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   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.
2. nadiasvertex commented on Apr 6, 2021

   

   nadiasvertex

   on Apr 6, 2021

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   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?
3. addaleax commented on Apr 6, 2021

   

   addaleax

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   @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.
4. nadiasvertex commented on Apr 6, 2021

   

   nadiasvertex

   on Apr 6, 2021

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   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!
5. addaleax commented on Apr 6, 2021

   

   addaleax

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   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)

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6. nadiasvertex commented on Apr 6, 2021

   

   nadiasvertex

   on Apr 6, 2021

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   That's very helpful, thanks!
7. nadiasvertex

   closed this as completedon Apr 6, 2021
8. RaisinTen commented on Apr 7, 2021

   

   RaisinTen

   on Apr 7, 2021

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   @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. 😕
9. addaleax commented on Apr 7, 2021

   

   addaleax

   on Apr 7, 2021

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   @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 ...).

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