How to "subclass" a node.js module

Question

I've been primarily a Perl coder for years, but also have a background in C++, so I'm coming from a "classical" OO background, and now learning node.js. I just read through The Principles of Object-Oriented JavaScript, which did a good job of explaining the JS concept of OO to classically-minded people like me. But I'm left with a question, specifically related to node.js and inheritance. Pardon me if I'm still using "classical" vocabulary to explain my problem.

Lets suppose I have a module lib/foo.js:

function foo() {
    console.log('Foo was called');
}

module.exports.foo = foo;

And I want to "subclass" this in another module lib/bar.js:

var foo = require('foo.js');

// Do some magic here with *.prototype, maybe?

function bar() {
    console.log('Bar was called');
}

module.exports.bar = bar;

Such that my main script can do this:

var bar = require('lib/bar.js');

bar.foo(); // Output "Foo was called"
bar.bar(); // Output "Bar was called"

Is this even possible? If so, what am I missing?

Or is this an anti-pattern? Or plain impossible? What should I do instead?

Jonathan Hall · Accepted Answer · 2014-10-11 00:39:49Z

2

Here's how I did it, to override one method in the request module. Warning: many node modules are poorly designed for extension, including request, as they do way too much stuff in the constructor. Not just a gazillion argument options, but starting up IO, connections, etc. For example, request does the http connection (eventually) as part of the constructor. There is no explicit .call() or .goDoIt() method.

In my example, I wanted to use querystring instead of qs to format forms. My module is cleverly named "MyRequest". In a separate file named myrequest.js you have:

var Request = require('request/request.js');
var querystring = require('querystring');

MyRequest.prototype = Object.create(Request.prototype);
MyRequest.prototype.constructor = MyRequest;

// jury rig the constructor to do "just enough".  Don't parse all the gazillion options
// In my case, all I wanted to patch was for a POST request 
    function MyRequest(options, callbackfn) {
        "use strict";
        if (callbackfn)
           options.callback = callbackfn;
        options.method = options.method || 'POST';          // used currently only for posts
        Request.prototype.constructor.call(this, options);  
           // ^^^  this will trigger everything, including the actual http request (icky)
           // so at this point you can't change anything more
    }

    // override form method to use querystring to do the stringify
    MyRequest.prototype.form = function (form) {
     "use strict";
        if (form) {
            this.setHeader('content-type', 'application/x-www-form-urlencoded; charset=utf-8');
            this.body = querystring.stringify(form).toString('utf8');
  // note that this.body and this.setHeader are fields/methods inherited from Request, not added in MyRequest.
            return this;
        }

        else
           return Request.prototype.form.apply(this, arguments);
    };

Then, in your application, instead of

var Request = require("request");
Request(url, function(err, resp, body)
  {
      // do something here
   });

you go

var MyRequest = require("lib/myrequest");
MyRequest(url, function(err, resp, body)
  {
      // do that same something here
   });

I'm not a JavaScript guru so there may be better ways...

edited Oct 11, 2014 at 0:39

Jonathan Hall

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answered Oct 10, 2014 at 23:08

user949300

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4 Comments

Jonathan Hall Over a year ago

Is this contained in a node.js module? AKA lib/myrequest.js or somesuch? Or does the code you pasted live in your main application?

Jonathan Hall Over a year ago

I think this is along the right track, and provides some good clues, but it doesn't quite work for me.

Jonathan Hall Over a year ago

I'm guessing that the 'request' module returns the object constructor, not an actual object, which is why your code works for you, but not for me. I had to use var foo = require('foo.js'); Bar.prototype = Object.create( foo.__proto__ ), but was able to get roughly the same result as you, after some trial and error. Thank you!

user949300 Over a year ago

@Flimzy correct. Often I wish the JS/node docs would be a bit more clear about what is a Class, what is a Constructor / factory, and what is an instance of an object.

Jonathan Hall · Accepted Answer · 2014-10-13 18:51:05Z

0

For reference, the specific solution I came up with to my sample code problem follows:

In lib/foo.js:

var Foo = function() {}

Foo.prototype.foo = function() {
    console.log('Foo was called!');
};

module.exports = new Foo;

In lib/bar.js:

var foo = require('./foo.js');

var Bar = function() {}

Bar.prototype = Object.create(foo.__proto__);
Bar.prototype.constructor = Foo;

Bar.prototype.bar = function() {
    console.log('Bar was called!');
};

module.exports = new Bar;

Then in my test script:

var bar = require('lib/bar.js');

bar.foo(); // Output "Foo was called"
bar.bar(); // Output "Bar was called"

answered Oct 13, 2014 at 18:51

Jonathan Hall

80.5k19 gold badges162 silver badges206 bronze badges

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