Handle errors and return the callback without throwing

[mapsam]

In #3 we are beginning to add checks for proper parameters and throwing errors if they are unexpected types. Currently we use the following approach:

if (!info[2]->IsFunction()) 
{
    Nan::ThrowTypeError("third arg 'callback' must be a function");
    return;
}

In an asynchronous function, this throws instead of returning an error in the callback, which is a super big benefit of a callback function. To avoid this currently, we can wrap async functions in try/catch statements but that gets a bit clumsy.

How can we better handle errors in the .cpp & Nan side of things so they are returned as errors instead of thrown?

cc @springmeyer @GretaCB

[mapsam]

Put together an elegant solution for this today with @springmeyer and @GretaCB. Instead of using Nan::ThrowTypeError() we can use Nan::MakeCallback. Here's the function for returning an error message as an error object to the callback:

function

inline void CallbackError(std::string message, v8::Local<v8::Function> callback) {
    v8::Local<v8::Value> argv[1] = { Nan::Error(message.c_str()) };
    Nan::MakeCallback(Nan::GetCurrentContext()->Global(), callback, 1, argv);
}

usage:

CallbackError("this is my error message", callback);

Important note - right now callback is defined as a v8::Local<v8::Value> so we'll have to update that to a v8::Local<v8::Function> to comply with the function above.

[mapsam]

Another important note - it's still useful to throw instead of return errors in some cases (i.e. callback function not provided) - so this type of error throwing will still exist for example purposes.