2011-03-26

I was recently playing around with node a command line javascript interpreter based on the V8 javascript engine and wanted/needed to implement a file downloader class that properly handles HTTP 302 Redirects.

After a bit of yelling and screaming, I had a working implementation
Enter the Javascript

var path require('path');
var url require('url');
var http require('http');
var fs require('fs');
var write_file;
//what global variable do we have?
var complete false;
var content_length 0;
var downloaded_bytes 0;

//create the downloader 'class'
var Downloader function() {

  //we will need to be able to set the remote file to download
  this.set_remote_file function(file) {
    remote_file file;
    local_file path.basenameremote_file );
  }
  
  //we want to set the local file to write to
  this.set_local_file function(file) {
    local_file file;
  }
//run this fukker!
  this.run function() {
    //start the download
    this.downloadremote_filelocal_file);
  }

  this.download function(remotelocalnum) {
    console.logremote );
    if num 10 ) {
      console.log'Too many redirects' );
    }
    //remember who we are
    var self this;
    //set some default values  
    var redirect false;
    var new_remote null;
    var write_to_file false;
    var write_file_ready false;
    //parse the url of the remote file
    var url.parse(remote);
    //set the options for the 'get' from the remote file
    var opts = {
      hostu.hostname,
      portu.port,
      pathu.pathname
    };
    //get the file
    var request http.get(optsfunction(response ) {
      switch(response.statusCode) {
        case 200:
          //this is good
          //what is the content length?
          content_length response.headers['content-length'];
          break;
        case 302:
          new_remote response.headers.location;
          self.download(new_remotelocal_filenum+);
          return;
          break;
        case 404:
          console.log("File Not Found");
        default:
          //what the hell is default in this situation? 404?
          request.abort();
          return;
      }
      response.on('data'function(chunk) {
        //are we supposed to be writing to file?
        if(!write_file_ready) {
          //set up the write file
          write_file fs.createWriteStream(local_file);
          write_file_ready true;
        }
        write_file.write(chunk);
        downloaded_bytes+=chunk.length;
        percent parseInt( (downloaded_bytes/content_length)*100 );
        console.log( percent );
      });
      response.on('end'function() {
        complete true;
        write_file.end();
      });
    });
    request.on('error'function(e) {
      console.log("Got error: " e.message);
    });
  }
}
exports.Downloader Downloader;

To use this, one would need to create a script that utilizes the class. For example, suppose we wanted to download an episode of Rathole Radio we would need to write something along the lines of the following:

var Downloader = require('./Downloader');
var dl = new Downloader.Downloader();
dl.set_remote_file('http://traffic.libsyn.com/ratholeradio/RR048_20_03_11.ogg');
dl.run();

Awesome 'Possum! Now quit reading and go do something nifty.

Comments
2011-03-27 jrobb:
nifty indeed!
2012-01-08 BenNG:
Thanks for the article :)
I was wondering something : the variable local in " this.download = function(remote, local, num) {...}" is never used right ?
2012-01-08 jezra:
@BenNG, haha! it certainly appears that way. The lines that actually write to the local file should probably be changed to use 'local' instead of 'local_file', or 'local' should be removed from the method.

It really comes down to scope preference.
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