Default export for synthetic module in V8

Question

I want to use default export with synthetic module in V8. I have synthetic_module, a module which exposes C++ functions to JS, and code like the following:

Local<String> txt = String::NewFromUtf8(isolate, u8R"(
  import defaultFoo from './myModule.js';
  defaultFoo();
").ToLocalChecked();

...

ScriptCompiler::Source src(txt, origin);
static Local<Module> module
  = ScriptCompiler::CompileModule(isolate, &src).ToLocalChecked();

// module->GetStatus() is kUninstantiated here

module->InstantiateModule(context,
  [](Local<Context> context,
     Local<String> specifier,
     Local<Module> referrer) -> MaybeLocal<Module> {
       return synthetic_module;
  });

// module->GetStatus() is still kUninstantiated
// if synthetic_module does not have default export

Using v8::TryCatch, I can get a SyntaxError that synthetic_module does not have default export. Is there a way to set default export when using synthetic module in V8? Thanks for your answer in advance.

score 0 · Accepted Answer · answered Dec 21 '19 at 06:56

The hint was in the error message. The error message was:

SyntaxError: The requested module './myModule.js' does not provide an export named 'default'

Therefore, to set the default export, I can just make a export with name "default" like this.

Local<Module> synthetic_module
  = Module::CreateSyntheticModule(
      isolate,
      String::NewFromUtf8(isolate, "Synthetic").ToLocalChecked(),
      { String::NewFromUtf8(isolate, "default").ToLocalChecked(), ... },
      [](Local<Context> context, Local<Module> module) -> MaybeLocal<Value> {
        auto isolate = context->GetIsolate();
        module->SetSyntheticModuleExport(
          String::NewFromUtf8(isolate, "default").ToLocalChecked(),
          Function::New(context, ...)
        );
        return MaybeLocal<Value>(True(isolate));
      }
    );

Default export for synthetic module in V8

1 Answers1