The Basics of Node.js Streams

Node.js is asynchronous and event driven in nature. As a result, it's very good at handling I/O bound tasks. If you are working on an app that performs I/O operations, you can take advantage of the streams available in Node.js. So, let's explore Streams in detail and understand how they can simplify I/O.

What are Streams

Streams are unix pipes that let you easily read data from a source and pipe it to a destination. Simply put, a stream is nothing but an EventEmitter and implements some specials methods. Depending on the methods implemented, a stream becomes Readable, Writable, or Duplex (both readable and writable). Readable streams let you read data from a source while writable streams let you write data to a destination.

If you have already worked with Node.js, you may have come across streams. For example, in a Node.js based HTTP server, request is a readable stream and response is a writable stream. You might have used fs module which lets you work with both readable and writable file streams.

Now that you know the basics, lets understand different type of streams. In this article, we will discuss readable and writable streams. Duplex streams are beyond the scope of this article.

Readable Stream

A readable stream lets you read data from a source. The source can be anything. It can be a simple file on your file system, a buffer in memory or even another stream. As streams are EventEmitters, they emit several events at various points. We will use these events to work with the streams.

Reading From Streams

The best way to read data from a stream is to listen to data event and attach a callback. When a chunk of data is available, the readable stream emits a data event and your callback executes. Take a look at the following snippet:

[js] var fs = require('fs'); var readableStream = fs.createReadStream('file.txt'); var data = ''; readableStream.on('data', function(chunk) { data+=chunk; }); readableStream.on('end', function() { console.log(data); }); [/js]

The function call fs.createReadStream() gives you a readable stream. Initially, the stream is in a static state. As soon as you listen to data event and attach a callback it starts flowing. After that, chunks of data are read and passed to your callback. The stream implementor decides how often data event is emitted. For example, an HTTP request may emit a data event once a few KB of data are read. When you are reading data from a file you may decide you emit data event once a line is read.

When there is no more data to read (end is reached), the stream emits an end event. In the above snippet, we listen to this event to get notified when the end is reached.

There is also another way to read from stream. You just need to call read() on the stream instance repeatedly until every chunk of data has been read.

[js] var fs = require('fs'); var readableStream = fs.createReadStream('file.txt'); var data = ''; var chunk; readableStream.on('readable', function() { while ((chunk=readableStream.read()) != null) { data += chunk; } }); readableStream.on('end', function() { console.log(data) }); [/js]

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by Sandeep Panda via SitePoint