2012-04-17

Node.js: expanding shortened URLs

This blog post explains how one can use Node.js to expand a URL that has been shortened by a service such as t.co (built into Twitter) and bit.ly. We’ll look at a simple implementation and at an advanced implementation that uses promises.

The minimum

You need to install Mikeal Rogers’ request module:
     npm install request
That module automatically follows all redirects from the shortened URL. Once you are at your final destination, you only need to find out where you are:
    var request = require("request");
    
    function expandUrl(shortUrl) {
        request( { method: "HEAD", url: shortUrl, followAllRedirects: true },
            function (error, response) {
                console.log(response.request.href);
            });
    }

Prettier with promises

If you want to write a function that returns the expanded URL, more work is needed. You have the option of using a callback, but promises usually lead to prettier code. Let’s use Kris Kowal’s Q module:
    npm install q
The “promising” code looks as follows.
    var Q = require("q");
    var request = require("request");
    function expandUrl(shortUrl) {
        return Q.ncall(request, null, {
            method: "HEAD",
            url: shortUrl,
            followAllRedirects: true
        // If a callback receives more than one (non-error) argument
        // then the promised value is an array. We want element 0.
        }).get('0').get('request').get('href');
    }
Node that the callback created by deferred.node() automatically handles errors. Invoking the function works like this:
    expandUrl("http://t.co/Zc3cUoly")
    .then(function (longUrl) {
        console.log(longUrl);
    });

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