how could get the "responseText" when request "XMLHttpRequest"

emscripten version: 3.1.7

i am trying to "XMLHttpRequest" communication.
and using Post Method.

try 1)
use EM_ASM_PTR

```
char* EmscriptenStr = (char*)EM_ASM_PTR({

var str = UTF8ToString($0);
var xhr = new XMLHttpRequest();

xhr.open('POST', str, true);
xhr.setRequestHeader("Content-Type", "text/plain; charset=UTF-8;");
xhr.send("");

xhr.onreadystatechange=function()
{
if (xhr.status==200)
{
console.log(xhr.responseText);
var lengthBytes = lengthBytesUTF8(xhr.responseText)+1;
var stringOnWasmHeap = _malloc(lengthBytes);
stringToUTF8(jsString, stringOnWasmHeap, lengthBytes);
return stringOnWasmHeap;

}
else
{
console.log(xhr.responseText +"\n");
console.log(xhr.status + "cannot find server\n")
}
}

}, URL.c_str());

std::string return_value = EmscriptenStr;

printf("UTF8 string says: %s\n", EmscriptenStr);
free(EmscriptenStr); // Each call to _malloc() must be paired with free(), or heap memory will leak!

return return_value;
```

but, error message : use of undeclared identifier 'var';

try 2)
use emscripten/val.h

```
EM_JS(void, Return_Callback, (EM_VAL val_handle), {
var value = Emval.toValue(val_handle);
value.onreadystatechange = function()
{
if (value.readyState==4 && value.status==200)
{
var msg = value.responseText;
console.log(msg);
}
};
});

std::string FunctionLua::RequestHistoryIndividualTag(std::string URL)
{
val xhr = val::global("XMLHttpRequest").new_();
xhr.call("open", std::string("POST"), std::string(URL));
xhr.call("setRequestHeader", std::string("Content-Type"), std::string("text/plain; charset=UTF-8;"));

std::string return_value = "";

xhr.call("send", std::string(""));

Return_Callback(xhr.as_handle());
return_value = xhr["responseText"].as();
printf("readyState:%d, status:%d, responseText:%s\n",xhr["readyState"].as(),xhr["status"].as(), return_value.c_str());

return return_value;
}
```

second try failed too,
i check the data arrived.
but it can't receive responseText data.