Really simple file uploads with Node.js and express

The upload form

This is the most obvious part of the challenge. You're probably familiar with this already. Anyway, for the sake of completeness of this article, here it is.
You will need a form in your browser for the file upload. I use Jade to generate my HTML and here how it looks:

form(action="...", method="post", enctype="multipart/form-data")
  input(type="file", name="displayImage")

The form.action will point to a route that handles the file upload. More below.

Accessing the uploaded file

If you're using recents versions of Node and Express, file uploads are a piece of cake. And I'll back this claim but before we go any further make sure you're familiar with routes, requests and responses in Express.
Okay, now let's justify the "piece of cake" claim. In our file upload route, the req parameter has req.files available. Here's an example of what the req.files would contain:

{
  displayImage: {
    size: 11885,
    path: '/tmp/1574bb60b4f7e0211fd9ab48f932f3ab',
    name: 'avatar.png',
    type: 'image/png',
    lastModifiedDate: Sun, 05 Feb 2012 05:31:09 GMT,
    _writeStream: {
      path: '/tmp/1574bb60b4f7e0211fd9ab48f932f3ab',
      fd: 14,
      writable: false,
      flags: 'w',
      encoding: 'binary',
      mode: 438,
      bytesWritten: 11885,
      busy: false,
      _queue: [],
      drainable: true
    },
    length: [Getter],
    filename: [Getter],
    mime: [Getter]
  }
}

In the req.files object above, the property displayImage is the name of the file field in your HTML form and req.files will contain one property each for every valid HTML file form field.
The file object contains the type, size and name properties for your server side validations.

Saving the uploaded file

Assuming the file is valid, you use the path property for the next step. The path would typically contain a location in the tmp folder. Your application logic could either require you to access the contents of the file or simply move the uploaded file to another location.

fs.readFile(req.files.displayImage.path, function (err, data) {
  // ...
  var newPath = __dirname + "/uploads/uploadedFileName";
  fs.writeFile(newPath, data, function (err) {
    res.redirect("back");
  });
});

In the fs.readFile callback, we have the data parameter through which we can access the contents of the file. The example above is taken from an application that needed to modify the file and save it in a new location. Thus fs.writeFile is used to write data to the newPath.
If your app needs to simply move the uploaded file without modifying the contents fs.rename can be used as more simpler option.

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That's all there is to it. I've done file uploads in many server side languages including Python, Java, Scala and PHP and I don't think its ever been this simple. So much for JavaScript being labeled as an inferior server side language.