How to call JS function from C (or get exported value from JS_Eval)?

[guyutongxue]

Hello! I'm new to QuickJS and I want to use it to embed my JS project into a C-API. But I'm encountering a problem that I don't know how to call JS functions in a JS_Evaled scripts. For example:

auto SOURCE = R"(
export function hello() {
  return 'Hello';
}
)" "\0"sv;

int main() {
  auto rt = JS_NewRuntime();
  auto ctx = JS_NewContext(rt);
  auto value = JS_Eval(ctx, SOURCE.data(), SOURCE.size() - 1, "main.js",  JS_EVAL_TYPE_MODULE);
  //   ^- Seems to be Promise<undefined> ?
  JSModuleDef* def = /* ??? */;
  auto ns = JS_GetModuleNamespace(ctx, def);
  auto fn = JS_GetPropertyStr(ctx, ns, "hello");
  auto result = JS_Call(ctx, fn, JS_UNDEFINED, 0, nullptr);
  auto str = JS_ToCString(ctx, result); // Expected "Hello"
  // [...]
  JS_FreeContext(ctx);
  JS_FreeRuntime(rt);
}

It's that possible to get the exported value from a JS_Evaled module? (The ??? part on above code) Or there is some other way to call JS functions from C code?

[bnoordhuis]

I can't guarantee it'll work forever but the return value from JS_Eval is the module, and that's just the wrapped JSModuleDef pointer:

JSValue v = JS_Eval(ctx, buf, len, filename, JS_EVAL_TYPE_MODULE);
if (JS_VALUE_GET_TAG(v) == JS_TAG_MODULE) {
    JSModuleDef *m = JS_VALUE_GET_PTR(v);
    // ...
}

[guyutongxue]

🤔 Thanks, but the JS_VALUE_GET_TAG(v) returns JS_TAG_OBJECT not JS_TAG_MODULE ...

[bnoordhuis]

Sorry, forgot to mention you also need to pass JS_EVAL_FLAG_COMPILE_ONLY to JS_Eval.

But, now that I think of it, you'll end up with a JSModuleDef that isn't linked or fully evaluated yet, so how useful that is remains to be seen.

[saghul]

You'd need to JS_EvalFunction the module and use JS_GetModuleNamespace to get the namespace object.

[guyutongxue]

Thank you @bnoordhuis and @saghul ! Here is a working example (sorry for using C++):

int main() {
  auto rt = JS_NewRuntime();
  auto ctx = JS_NewContext(rt);

  auto bytecode = JS_Eval(ctx, SOURCE.data(), SOURCE.size() - 1, "main.js",
                          JS_EVAL_TYPE_MODULE | JS_EVAL_FLAG_COMPILE_ONLY);
  JS_ResolveModule(ctx, bytecode);
  JS_EvalFunction(ctx, bytecode);
  auto def = static_cast<JSModuleDef*>(JS_VALUE_GET_PTR(bytecode));
  auto ns = JS_GetModuleNamespace(ctx, def);

  auto fn = JS_GetPropertyStr(ctx, ns, "hello");
  auto result = JS_Call(ctx, fn, JS_UNDEFINED, 0, nullptr);
  auto str = JS_ToCString(ctx, result);
  std::cout << str << std::endl;  // Expected "Hello"

  JS_FreeContext(ctx);
  JS_FreeRuntime(rt);
}

Use JS_EVAL_FLAG_COMPILE_ONLY to get a JSModuleDef and use JS_ResolveModule (I'm not sure if this is necessary but anyway it works) and JS_EvalFunction to fully load this module. Then JS_GetModuleNamespace works like a charm.

[saghul]

great! Yeah I think you can remove JS_ResolveModule it's likely called internally anyway, but I'm not 100% sure.

[dbalabka]

@saghul, how do we await the function result if the function is async? Thanks in advance!

export async function hello() {
  return 'Hello';
}

[bnoordhuis]

The return value when you call an async function is a promise. Call JS_ExecutePendingJob until JS_PromiseState no longer returns JS_PROMISE_PENDING.