Using Q promises with node.js http requests

This post is intended at providing a code snippet on how to use Q promises with node.js http requests.

I was introduced to the concept of promises and deferred when I found myself uncomfortable writing asynchronous functions within callbacks.

The code is not:
1. Maintanable
2. Readable
3. Intuitive

That is when I stumbled on JQuery Deffered and Promises. Once I got a hang of it I came across the Q Library that does pretty much the same thing with node.js.

I found it difficult to apply it to node.js http requests and after I gave it more time I got a hang of it.

The following gist demonstrates on how to wrap the the node.js http request with the q library.

In the gist we are attempting to invoke the Google News API using the node.js http requests.

	//import the http library
	var http = require('http'),
	//npm install q before requiring it
	Q = require('q');
	//a js object with options
	var googleNewsOptions = {
	hostname: 'ajax.googleapis.com',
	path: '/ajax/services/search/news?v=1.0&q=nodejs',
	method: 'GET'
	};
	/**
	* wrapper for http request object
	* @param {Object} requestOptions
	* @return Promise Object
	*/
	function promisedRequest(requestOptions) {
	//create a deferred object from Q
	var deferred = Q.defer();
	var req = http.request(requestOptions, function(response) {
	//set the response encoding to parse json string
	response.setEncoding('utf8');
	var responseData = '';
	//append data to responseData variable on the 'data' event emission
	response.on('data', function(data) {
	responseData += data;
	});
	//listen to the 'end' event
	response.on('end', function() {
	//resolve the deferred object with the response
	deferred.resolve(responseData);
	});
	});
	//listen to the 'error' event
	req.on('error', function(err) {
	//if an error occurs reject the deferred
	deferred.reject(err);
	});
	req.end();
	//we are returning a promise object
	//if we returned the deferred object
	//deferred object reject and resolve could potentially be modified
	//violating the expected behavior of this function
	return deferred.promise;
	};
	//incoking the above function with the request options
	promisedRequest(googleNewsOptions)
	.then(function(newsResponse) { //callback invoked on deferred.resolve
	console.log(JSON.stringify(newsResponse));
	}, function(newsError) { //callback invoked on deferred.reject
	console.log(newsError);
	});

view raw promisedRequest.js hosted with ❤ by GitHub