How do I export an object with added prototype methods in node.js

Question

I have an object in a file and want to be able to require that file and then create new instances of the object at whim, but I've hit a snag. This seems so incredibly basic, what am I missing.

hat.js

function Hat(owner) {
    this.owner = owner;
}
Hat.prototype.tip = function() {
    console.log("and he (" + owner + ") tipped his hat, just like this");
}
exports.Hat = Hat;

node terminal

Attempt 1

> require('./hat.js');
> var mighty_duck = new Hat('Emilio');
  ReferenceError: Hat is not defined

Attempt 2

> var Hat = require('./hat.js');
> var mighty_duck = new Hat('Emilio');
  { owner: 'Emilio' }
> mighty_duck.tip();
  TypeError: Object #<Hat> has no method 'tip'

Edit

I, most unfortunately, left out what turned out to be the biggest problem bit. The fact that I was trying to use

util.inherits(Hat, EventEmitter);

So my hat.js would actually be

function Hat(owner) {
    this.owner = owner;
}
Hat.prototype.tip = function() {
    console.log("and he (" + owner + ") tipped his hat, just like this");
}
util.inherits(Hat, EventEmitter);
exports.Hat = Hat;

This is an issue, because, apparently it is important to have your inherits call before you start extending the prototype. The fix is simple, move the inherits call up a few lines

function Hat(owner) {
    this.owner = owner;
}
util.inherits(Hat, EventEmitter);
Hat.prototype.tip = function() {
    console.log("and he (" + owner + ") tipped his hat, just like this");
}
exports.Hat = Hat;

Dziad Borowy · Accepted Answer · 2012-11-13 17:03:14Z

4

Have a look here: http://openmymind.net/2012/2/3/Node-Require-and-Exports/

you might try this instead:

function Hat(owner) {
    this.owner = owner;
}
Hat.prototype.tip = function() {
    console.log("and he (" + this.owner + ") tipped his hat, just like this");
}
module.exports = Hat;

and then sth. like this:

var hat = require('./hat.js');
var mighty_duck = new hat('Emilio');
mighty_duck.tip()

edited Nov 13, 2012 at 17:03

answered Nov 13, 2012 at 16:40

Dziad Borowy

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2 Comments

Chad Over a year ago

setting exports does not work, try doing this and requiring the file in the repl. You get an empty object. If you want to overwrite the object in exports you need to do module.exports = exports = ....

Dziad Borowy Over a year ago

@Chad Yes, you're right. I've corrected my answer, so it works now. Thanks!

Chad · Accepted Answer · 2012-11-13 16:43:03Z

2

You are doing your require wrong:

var Hat = require('./hat.js').Hat;

Is what you want. When you do exports.Hat = Hat; you are exporting an object (exports) with a propery of Hat. So when you require './hat.js' you get the object, and need to access the Hat property.

From node repl:

> require('./hat.js');
{ Hat: [Function: Hat] }
> require('./hat.js').Hat;
[Function: Hat]
> var Hat = require('./hat.js').Hat;
undefined
> var mighty_duck = new Hat('Emilio');
undefined
> mighty_duck.tip();
and he (Emilio) tipped his hat, just like this

Of course that was causing an error since you said owner and not this.owner in tip(), but after I changed that it worked fine :)

If you want to just do

var Hat = require('./hat.js'),
    hat = new Hat('Emilio');

Then change your export to:

module.exports = exports = Hat;

And it works:

> require('./hat.js');
[Function: Hat]
> var Hat = require('./hat.js');
undefined
> var hat = new Hat('Emilio');
undefined
> hat.tip();
and he (Emilio) tipped his hat, just like this

EDIT Cleaned up and inherits from EventEmitter:

var util = require('util'),
    events = require('events');

var Hat = exports.Hat = function(owner) {
    events.EventEmitter.call(this);
    this.owner = owner;
}

util.inherits(Hat, events.EventEmitter);

Hat.prototype.tip = function() {
    console.log("and he (" + this.owner + ") tipped his hat, just like this");
}

edited Nov 13, 2012 at 16:43

answered Nov 13, 2012 at 16:35

Chad

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4 Comments

wmarbut Over a year ago

Thanks, that works exactly correctly. Unfortunately for me, I left out the problem bit (that I just discovered) in my example code. I was trying to extend EventEmitter.

Chad Over a year ago

extending EventEmitter doesn't change how you export; I added some cleaned up code that inherits from event emitter.

wmarbut Over a year ago

thanks for your answer. Your are correct that extending it doesn't change how you export, but it does kill your prototype if you don't call the inherits first. An odd quirk. Cheers!

Chad Over a year ago

Right it inherits via Hat.prototype = new events.EventEmitter; or something similar which will set your prototype, overwriting anything that is there.

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