src: precompute field names

Field names do not change from row to row, so let's create JS strings
only during the process of calling back into JS with the first row, not
with each row. We save the JS strings we created while calling back
with the first row, and re-use them when creating objects for
subsequent rows.

Re: nodejs/abi-stable-node#458

src/statement.cc

@@ -587,8 +587,16 @@ void Statement::Work_AfterAll(napi_env e, napi_status status, void* data) {
                 Napi::Array result(Napi::Array::New(env, baton->rows.size()));
                 auto it = static_cast<Rows::const_iterator>(baton->rows.begin());
                 decltype(it) end = baton->rows.end();
+                std::vector<napi_value> js_names;
                 for (int i = 0; it < end; ++it, i++) {
-                    (result).Set(i, RowToJS(env, it->get()));
+                    Row* row = it->get();
+                    if (js_names.empty()) {
+                        for (auto& field : *row) {
+                            const napi_value js_field_name = Napi::String::New(e, field->name);
+                            js_names.push_back(js_field_name);
+                        }
+                    }
+                    (result).Set(i, RowToJS(env, row, js_names));
                 }
 
                 Napi::Value argv[] = { env.Null(), result };
@@ -711,8 +719,16 @@ void Statement::AsyncEach(uv_async_t* handle) {
             Napi::Value argv[2];
             argv[0] = env.Null();
 
-            for(auto& row : rows) {
-                argv[1] = RowToJS(env,row.get());
+            std::vector<napi_value> js_names;
+            for(auto& rowRef : rows) {
+                Row* row = rowRef.get();
+                if (js_names.empty()) {
+                    for (auto& field : *row) {
+                        const napi_value js_field_name = Napi::String::New(env, field->name);
+                        js_names.push_back(js_field_name);
+                    }
+                }
+                argv[1] = RowToJS(env, row, js_names);
                 async->retrieved++;
                 TRY_CATCH_CALL(async->stmt->Value(), cb, 2, argv);
             }
@@ -787,11 +803,12 @@ void Statement::Work_AfterReset(napi_env e, napi_status status, void* data) {
     STATEMENT_END();
 }
 
-Napi::Value Statement::RowToJS(Napi::Env env, Row* row) {
+Napi::Value Statement::RowToJS(Napi::Env env, Row* row, const std::vector<napi_value>& js_names) {
     Napi::EscapableHandleScope scope(env);
 
     auto result = Napi::Object::New(env);
 
+    size_t js_name_idx = 0;
     for (auto& field : *row) {
 
         Napi::Value value;
@@ -816,7 +833,10 @@ Napi::Value Statement::RowToJS(Napi::Env env, Row* row) {
             } break;
         }
 
-        result.Set(field->name, value);
+        napi_value field_name = js_names.empty()
+            ? Napi::String::New(env, field->name)
+            : js_names[js_name_idx++];
+        result.Set(field_name, value);
     }
 
     return scope.Escape(result);

src/statement.h

@@ -220,7 +220,7 @@ class Statement : public Napi::ObjectWrap<Statement> {
     bool Bind(const Parameters &parameters);
 
     static void GetRow(Row* row, sqlite3_stmt* stmt);
-    static Napi::Value RowToJS(Napi::Env env, Row* row);
+    static Napi::Value RowToJS(Napi::Env env, Row* row, const std::vector<napi_value>& js_names = std::vector<napi_value>());
     void Schedule(Work_Callback callback, Baton* baton);
     void Process();
     void CleanQueue();