Node.js Tutorial - Node.js Custom Streams

To create our own streams, inherit from the stream class and implement a few base methods listed in the following table.

Use-case	Class	Method(s) to Implement
Reading only	Readable	_read
Writing only	Writable	_write
Reading and writing	Duplex	_read, _write
Operate on read data and write the result	Transform	_transform, _flush

Creating a Readable Stream

We can inherit from Readable class, implement the _read member in your class.

The method is called by the stream API internally when someone requests data to be read.

To pass data on, call the inherited member function push passing in the data.

If you call push(null), this signals the end of the read stream.

The following code shows how to create a readable stream and return 1-10.

If you run this, you will see all these numbers printed.


var Readable = require('stream').Readable; 
var util = require('util'); 
/*  w w  w  .j  ava  2 s.  co m*/
function Counter() { 
    Readable.call(this); 
    this._max = 10; 
    this._index = 1; 
} 
util.inherits(Counter, Readable); 

Counter.prototype._read = function () { 
    var i = this._index++; 
    if (i > this._max)
        this.push(null);
    else { 
        var str = ' ' + i; 
        this.push(str);
    } 
}; 
// Usage, same as any other readable stream 
var counter = new Counter(); 
counter.pipe(process.stdout);

Creating a Writable Stream

You inherit from the Writable class and implement the _write method.

The _write method is passed in a chunk that needs processing as its first argument.

The following code creates a simple writable stream that logs to the console all the data passed in.


var Writable = require('stream').Writable; 
var util = require('util'); 
/*from w w w. j  a v  a 2 s  .  co  m*/
function Logger() {
     Writable.call(this); 
} 
util.inherits(Logger, Writable); 

Logger.prototype._write = function (chunk) {
     console.log(chunk.toString()); 
}; 

// Usage, same as any other Writable stream 
var logger = new Logger(); 

var readStream = require('fs').createReadStream('log.txt'); 
readStream.pipe(logger);