Handling asynchronous URL parsing in NodeJS

Question

I'm pretty new to Node.js and Javascript, so please forgive my confusion about the callback mechanism Bacchanalia.

My problem

I am building a simple Node.js application, which receives HTTP requests and sends their JSON body payload combined with their URL arguments as an input to a function. For the sake of learning I do not use any external framework or npm package.

So basically,

curl -i \
     -H "Content-Type: application/json" \
     -X POST \
     -d '{"jsonKey":"jsonValue"}' \
     'http://localhost:8888/y?urlKey=urlValue'

Should call an arbitrary function f with the parameters f({"jsonKey":"jsonValue", "urlKey":"urlValue"}).

Converting the URL args is pretty straightforward and requires no callbacks at all:

/* Parse the URL arguments */
function parseArguments(request) {
    var url_obj = url.parse(request.url, true);
    return url_obj.query;
}

The plot thickens when I try to add the JSON Payload to the returned value:

/* Parse the URL arguments and the JSON payload */
function parseArguments(request) {
    // URL Parameters 
    var arguments_as_object = url.parse(request.url, true).query;

    // JSON Payload, if any
    var body = [];
    request.on('data', function(chunk) {
        body.push(chunk);
    }).on('end', function() {
        body = Buffer.concat(body).toString();
        if (body) {
            body = JSON.parse(body);
            Object.assign(arguments_as_object, body);
        }
        console.log(new Date(), "Arguments with JSON are ", arguments_as_object);
    });

    console.log(new Date(), "Return value is ", arguments_as_object);
    return arguments_as_object;
}

The logs are:

2017-04-16T13:14:47.807Z 'Return value is ' { a: 'b' }
2017-04-16T13:14:47.826Z 'Arguments with JSON are ' { a: 'b', jsonKey: 'jsonValue' }

Unsurprisingly, parseArguments returns before the JSON parse is complete due to the asynchronous nature of javascript.

My question

How can I get the results of the asynchronous call and use them outside the function? Is there a way other than sending a callback function with all its arguments?

What have I tried

SFTW.
Using a callback instead of a return value. This seems to be the right way, but then I would have to send all the arguments of the callback to parseArguments, which would add a lot of unnecessary variables to its signature.

You can't. An async operation won't be completed before your function returns so there's no way to return an async result from your function. You will have to use a callback or a promise. See the [marked duplicate](http://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call) for various choices. I would recommend returning a promise from your function and then the caller can use `.then()` on the promise to get the async result. — jfriend00, Apr 16 '17 at 13:40

score 1 · Accepted Answer · answered Apr 16 '17 at 13:40

I think you've more or less answered your own question. You have to use a callback instead of returning a value.

You can somewhat address your concern about sending all of the callback arguments to parseArguments by wrapping that function in another function:

function mainFunc = (a, b, c, jsonData) {
...
}    
function parseCb = (jsonData) {
mainFunc(1,2,3, jsonData)
}
parseArguments(request, parseCb)

You can also inject a level of clarity to your asynchronous code by implementing promises, which can be returned and passed around like synchronous variables:

function parseArguments(request) {

return new Promise((resolve, reject) => {
    // URL Parameters 
    var arguments_as_object = url.parse(request.url, true).query;

    // JSON Payload, if any
    var body = [];
    request.on('data', function(chunk) {
        body.push(chunk);
    }).on('end', function() {
        body = Buffer.concat(body).toString();
        if (body) {
            body = JSON.parse(body);
            Object.assign(arguments_as_object, body);
           resolve(arguments_as_object)
        }
        console.log(new Date(), "Arguments with JSON are ", arguments_as_object);
    });

    console.log(new Date(), "Return value is ", arguments_as_object);
    return arguments_as_object;
}
})

Handling asynchronous URL parsing in NodeJS

My problem

My question

What have I tried

1 Answers1