Inheriting Functionality from Other Objects

The util Module provides the util.inherits() method to allow you to create objects that inherit the prototype methods from another object. When you create a new object, the prototype methods are automatically used. You have already seen this in a few examples in this book—for example, when implementing your own custom Readable and Writable streams.

The following is the syntax of the util.inherits() method:

Click here to view code image

util.inherits(constructor,  superConstructor)

The prototype constructor is set to the prototype superConstructor and is executed when a new object is created. You can access superConstructor from your custom object constructor by using that constructor.super_ property.

Listing 10.2 illustrates using inherits() to inherit the events.EventEmitter object constructor to create a Writable stream. Notice that on line 11 the object is an instance of events.EventEmitter. Also notice that on line 12 the Writer.super_ value is eventsEmitter. Figure 10.2 shows the results of Listing 10.2.

Listing 10.2 util_inherit.js: Using inherits() to inherit the prototypes from event.EventEmitter

Click here to view code image

01 var util = require("util");
02 var events = require("events");
03 function Writer() {
04   events.EventEmitter.call(this);
05 }
06 util.inherits(Writer, events.EventEmitter);
07 Writer.prototype.write = function(data) {
08   this.emit("data", data);
09 };
10 var w = new Writer();
11 console.log(w instanceof events.EventEmitter);
12 console.log(Writer.super_ === events.EventEmitter);
13 w.on("data", function(data) {
14     console.log('Received data: "' + data + '"'),
15 });
16 w.write("Some Data!");

Figure 10.2 Output from implementing inherits() to build a Writable stream.