Your first Node.js module

http://cnnr.me/b/2012/05/your-first-node-dot-js-module/

by c_t_montgomery • 14 years ago

74 23 14 years ago

tferris3• 14 years ago

Slightly OT: does anyone have a good explanation for module.exports vs exports and for the expression 'exports = module.exports = someFunction' often found in modules?

mattyb• 14 years ago

According to the documentation, they are the same:

http://nodejs.org/docs/v0.6.18/api/modules.html#modules_the_...

strager• 14 years ago

Reassigning the `exports` variable does not change the return value of `require`.

The return value of `require` is `module.exports`, which may be reassigned.

Initially, the global `exports` variable is set to the same value as `module.exports`. Writing `exports = module.exports = ...` ensures they have the same value as you would expect.

tferris3• 14 years ago

> Writing `exports = module.exports = ...` ensures they have the same value as you would expect.

But where should they have the same value? I thought require returns just module.exports if it exists and ignores exports.

smagch2• 14 years ago

=== hoge.js ===

module.exports = exports = { hello: 'hello', world: 'world' };

exports.yetAnother = 'yet another exports key';

===

and this yield

require('./hoge') -> {

   hello: 'hello',

   world: 'world',

   yetAnother: 'yet another exports key'

 }

But, when hoge.js is

=== hoge.js ===

module.exports = { hello: 'hello', world: 'world' };

exports.yetAnother = 'yet another exports key';

===

require('./hoge') -> // { hello: 'hello', world: 'world' }

=== hoge.js ===

exports = { hello: 'hello', world: 'world' };

exports.yetAnother = 'yet another exports key';

===

require('./hoge') -> // {}

tferris3• 14 years ago

So you reassigned module.exports to exports in order to get all objects assigned to both module.exports and exports otherwise objects assigned to module.exports would override those assigned to exports which would stay ignored, right?

But: to get all objects do I have to assign module.exports to exports (so change the order from your first example)?:

exports = module.exports = { hello: 'hello', world: 'world' };

radagaisus3• 14 years ago

Also, `this` is exported in modules.

zeekay• 14 years ago

I've noticed that, and have been taking advantage of it. Any downside to using `this`, besides potential scoping issues?

tgasson• 14 years ago

Every module will have a `module` variable available to it.

    `module` defaults to {
        id: ...
        exports: {}
        parent:...
        filename:...
        loaded:...
        exited:...
        children:...
        paths:...
    }

Whatever module.exports is at the end of the module is passed to the require statement.

Every file essentially has two lines at the top.

    this = module.exports;
    var exports = module.exports;

The first helps lazy developers by allowing exporting without explicitly saying so. Because of this, the following alone is a valid module that exports {foo:'bar'}

    foo = 'bar'; // can also be written as this.foo = 'bar';

And the second matches the CommonJS standard. So the following CommonJS module will work:

    exports.foo = 'bar';

If you assign module.exports at any time after using `this` or `exports`, module.exports will point to the new object, and anything on the original module.exports object wont be passed to the `require` call.

tg3• 14 years ago

The error that is thrown towards the end of this example should probably be passed back to the callback as an argument (e.g. callback(error, result)) instead.

Throwing errors in an asynchronous context can have messy results for trying to handle them. http://benno.id.au/blog/2011/08/08/nodejs-exceptions is a good explanation.

c_t_montgomery5• 14 years ago

Great point. I'll update the gist. Thanks for bringing this up!

secoif3• 14 years ago

It should also be pointed out you rarely need to edit the package.json manually:

npm init: Prompts for basic/common package.json entries

npm install --save: Install a module and place into dependencies

npm install --save-dev: Install a module and place into dev dependencies

npm install --save-optional: Install a module and place into optional dependencies

http://blog.timoxley.com/post/22371787222/handy-new-options-...

c_t_montgomery5• 14 years ago

I was debating whether or not to put it in there. Since Isaac mentions it in his article that I link to, I figured most people would see it there. Very valid point though. I think I may edit the post and put it up there. Thanks!

c_t_montgomery5• 14 years ago

...and the post has been updated. Thanks again.

phase_9• 14 years ago

I liked the part where you encouraged the developer to think about how the API will be consumed by the end user; however even a brief mention of some BDD / Unit tests here would have been nice (:

c_t_montgomery5• 14 years ago

I agree it would have been nice. Apologies for not including it.

aeurielesn• 14 years ago

Just for the record, I think you should have used `encodeURIComponent` instead of `encodeURI`. And in `exports.info` as well.

c_t_montgomery5• 14 years ago

Ah great catch. I just changed it. Thanks!

Your first Node.js module

Related Stories