Transform Streams

A Transform stream extends the Duplex stream but modifies the data between the Writable stream and the Readable stream. This stream type can be extremely useful when you need to modify data from one system to another. Some examples of Transform streams are:

zlib stream

crypto streams

A major difference between the Duplex and the Transform streams is that for Transform streams, you do not need to implement the _read() and _write() prototype methods. These are provided as pass-through functions. Instead, you implement the _transform(chunk, encoding, callback) and _flush(callback) methods. The _transform() method should accept the data from write() requests, modify it, and push out the modified data.

The code in Listing 5.9 illustrates the basics of implementing a Transform stream. The stream accepts JSON strings, converts them to objects, and then emits a custom event named object that sends the object to any listeners. The _transform() function also modifies the object to include a handled property and then sends on a string form. Notice that lines 18–21 implement the object event handler function, which displays certain attributes. Figure 5.9 shows the output for Listing 5.9. Notice that the JSON strings now include the handled property.

Listing 5.9 stream_transform.js: Implementing a Transform stream object

Click here to view code image

01 var stream = require("stream");
02 var util = require("util");
03 util.inherits(JSONObjectStream, stream.Transform);
04 function JSONObjectStream (opt) {
05   stream.Transform.call(this, opt);
06 };
07 JSONObjectStream.prototype._transform = function (data, encoding, callback) {
08   object = data ? JSON.parse(data.toString()) : "";
09   this.emit("object", object);
10   object.handled = true;
11   this.push(JSON.stringify(object));
12   callback();
13 };
14 JSONObjectStream.prototype._flush = function(cb) {
15   cb();
16 };
17 var tc = new JSONObjectStream();
18 tc.on("object", function(object){
19   console.log("Name: %s", object.name);
20   console.log("Color: %s", object.color);
21 });
22 tc.on("data", function(data){
23   console.log("Data: %s", data.toString());
24 });
25 tc.write('{"name":"Carolinus", "color": "Green"}'),
26 tc.write('{"name":"Solarius", "color": "Blue"}'),
27 tc.write('{"name":"Lo Tae Zhao", "color": "Gold"}'),
28 tc.write('{"name":"Ommadon", "color": "Red"}'),

Figure 5.9 Output of stream_transform.js, implementing a custom Transform object.