adding an async function called "shout"

[mapsam]

Adds an asynchronous function called shout. Here's what it looks like on the JS side:

var HW = new HelloWorld();
HW.shout('hey', { louder: true }, function(err, shout) {
  if (err) throw err;
  console.log(shout); // => hey!
});

Next up:

• handle different options
• return errors instead of throwing Handle errors and return the callback without throwing #4

cc @springmeyer @GretaCB

[mapsam]

I would expect err to be here, but we are throwing within hello_world.cpp instead of returning an error message. Not sure how to do this. @springmeyer

[mapsam]

Actually on second thought, maybe throwing is best here and the test should be written differently. Not sure.

[springmeyer]

Right, the current code potentially both throws and returns an error in the callback. So the test is incorrect and should do a try/catch instead and assert the catch is hit.

This "two ways to catch errors" is not ideal and results in extra code in JS of course. But it is not viable to do the validation in the HelloWorld::AsyncShout because one cannot safely copy the data into the threadpool without validating.

Potentially one way to avoid going into the threadpool/still return all errors in the callback would be to call the callback inside HelloWorld::shout instead of throwing errors. I think this should work but I've not tried it. Would be a great thing to look into for this repo because figuring out a cleaner way to do this would benefit the usability of all modules based on this code. Can you ticket for a little later?

[mapsam]

@springmeyer seeing that the return_string now comes back empty for this method after wrapping it all within the try/catch. Seems odd, but if I pull it out:

/***************** custom code here ******************/

std::string return_string = baton->phrase + "!";

if (baton->louder)
{
    return_string += "!!!!";
}

/***************** end custom code *******************/

try
{
    baton->result = return_string;
}
catch (std::exception const& ex)
{
    baton->error_name = ex.what();
}

things work fine

[mapsam]

All green @springmeyer - this is ready to be merged.

I also added the default option to the Makefile, which just executes npm install --build-from-source

[springmeyer]

Great, looks good to me to merge as well - we can address future improvements as they arise. Notably integrating the results of Project-OSRM/node-osrm#193 (comment), when I understand more.