Processing Files

NodeJS includes another useful module called "fs" (which is short for File System). Using it we can read and write files. Reading and writing files is very slow. NodeJS wants to appear to be very fast and responsive. It is possible to read and write files both asynchrounously and synchronously. As much as possible, we want to read and write files asynchronously.

We can write a file asynchronously thus:

var fs = require("fs");
var fileName = "/desktop/file.txt";
var content = "99 glasses of margaritas on the wall.";
fs.writeFile(fileName, content, function(err) {
    // only when there's an error ...
});

We can read a file asynchronously thus:

var fs = require("fs");
var fileName = "/desktop/file.txt";
fs.readFile(fileName, function(err, data) {
    if (err) {
        // only when there's an error ...
    }
    else {
        var content = data.toString();
        // ...
    }
});

HTTP URLs are different than file system paths. The way you locate a resource via HTTP is not the same as you would on the local file system. Sometimes we may need to convert a URL path to something equivalent on the local file system. Suppose we have the following directory structure:

/root
    /server
        /web
            index.html
            about.html
            contact.html
            ...
        ...
    ...

When the client requests the HTTP path (URL): http://example.com/about.html then the server has to somehow map the URL to the actual file. The built in path package serves this purpose.

var http = require("http");
var path = request("path");
var url = require("url");
var server = http.createServer(function(request, response) {
    var uri = url.parse(request.url).pathname;
    var filename = path.join(__dirname, "./web/" + uri);
    // ...
});
var port = 8080;
server.listen(port);

You'll notice the __dirname variable above. It is a built-in global variable that NodeJS provides to gives the directory name of the currently executing script (*.js) file. We can use that as a basis with which to form a valid file system path. The above example assumes the script is running beneath the /root/server/ directory listed before. Any valid file path can be used, such as ../../../<file> to change to a higher level directory, and so on.

Exercises

• When reading a file, why does data have to be converted to a string? Under what circumstances do you NOT want to convert it to a string?

• Create a response that returns an image file when the user browses to it.

• Write code that determines whether a directory exists and creates one if it does not.