Node.JS - Fundamentals of Asynchronous Programming, Part 1

Now, after the release of the stable version of Node.JS 0.2.0 , I decided to start a series of articles on programming with its use.

The main concept of Node.JS is that any I / O operations are implemented by default as asynchronous, after the operation, a callback function will be called, the first parameter of which will be an error or null.

Hide asynchronous nesting

Suppose we need to create a directory, including all its parents. And only if it was possible to create it, start writing to this directory.
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In order to hide the complexity of working with asynchronous operations, we move the work to create the directory into a separate asynchronous method:

var path = require( 'path' );
var fs = require( 'fs' );

var mkdir_p = function (pth, callback)
{
fs.stat(pth, function (err, stat)
{
if (!err && stat)
{
callback( null );
return ;
}
mkdir_p(path.dirname(pth), function (err)
{
if (err)
{
callback(err);
return ;
}
fs.mkdir(pth, 0755, callback);
});
});
};

* This source code was highlighted with Source Code Highlighter .

The function first checks for the presence of a directory, and if it exists, calls the callback function without error. In case the directory is missing, mkdir_p is called for the parent directory, and then the desired directory is created.

As you can see, it was possible to successfully implement recursion, and transmitting the error information to the callback function as standard for Node.JS.

We use, for example, like this:

mkdir_p(path.dirname(outPath), function (err)
{
if (err)
{
response.writeHead(500, { 'Content-Type' : 'text/plain' });
response.end( 'Cannot create directory' );
return ;
}
getFile(url, inPath, function (err)
{
if (err)
{
response.writeHead(500, { 'Content-Type' : 'text/plain' });
response.end( 'Cannot read file' );
return ;
}
transcode(inPath, outPath, function (err)
{
if (err)
{
response.writeHead(500, { 'Content-Type' : 'text/plain' });
response.end( 'Cannot transcode file' );
return ;
}
sendRemoveFile(outPath, response, function (err)
{
if (err)
{
console.log( 'Cannot send file' );
return ;
}
});
});
});
});

* This source code was highlighted with Source Code Highlighter .

In the next article I will tell you about working with external processes.

• Node.JS - form the resulting document using other HTTP sources
• Node.JS - Fundamentals of Asynchronous Programming, Part 1