In Node.js world, we usually encapsulate a service into a module, which means the module need to export the façade of the service. In most case the service could be a singleton, all apps use the same service.

But in some rare cases, people might would like to create several instances of the service ,which means the module also need to also export the service constructor.

A very natural idea is to export the default service, and expose the constructor as a method of the default instance. So we could consume the service in this way:

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var defaultService = require('service');
var anotherService = service.newService();

So we need to write the module in this way:

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function Service() { }
module.exports = new Service();
moudle.exports.newService = Service;

But for some reason, node.js doesn’t allow module to expose object by assigning the a object to module.exports.
To export a whole object, it is required to copy all the members of the object to moudle.exports, which drives out all kinds of tricky code.
I misunderstood how node.js require works, and HERE is the right understanding. Even I misunderstood the mechanism, but the conclusion of this post is still correct. To export function is still a more convenient way to export both default instance and the constructor.

And things can become much worse when there are backward reference from the object property to itself.
So to solve this problem gracefully, we need to change our mind.
Since it is proved that it is tricky to export a object, can we try to expose the constructor instead?

Then answer is yes. And Node.js does allow we to assign a function to the module.exports to exports the function.
So we got this code.

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function Service() { }
module.exports = Service;

So we can use create service instance in this way:

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var Service = require('service');
var aService = new Service();

As you see, since the one we exported is constructor so we need to create a instance manually before we can use it. Another problem is that we lost the shared instance between module users, and it is a common requirement to share the same service instance between users.

How to solve this problem? Since as we know, function is also kind of object in javascript, so we can kind of add a member to the constructor called default, which holds the shared instance of the service.

This solution works but not in a graceful way! A crazy but fancy idea is that can we transform the constructor itself into kind of singleton instance??!! Which means you can do this:

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var defaultService = require('service');
defaultService.foo();
var anotherService = service();
anotherService.foo();

The code style looks familiar? Yes, jQuery, and many other well-designed js libraries are designed to work in this way.
So our idea is kind of feasible but how?

Great thank to Javascript’s prototype system (or maybe SELF’s prototype system is more accurate.), we can simply make a service instance to be the constructor’s prototype.

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function Service() { }
module.exports = Service;
Service.__proto__ = new Serivce;

Sounds crazy, but works, and gracefully! That’s the beauty of Javascript.

Javascript is weakly typed, and its type system always behaves different than your expectation.
Javascript provide typeof operator to test the type of a variable. it works fine generally. e.g.

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typeof(1) === 'number'
typeof('hello') === 'string'
typeof({}) === 'object'
typeof(function(){}) === 'function'

But it is not enough, it behaves stupid when you dealing with objects created by constructors. e.g.
if you expected

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typeof(new Date('2012-12-12')) === 'date' // Returns false

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typeof(new Date('2012-12-12')) === 'object' // Returns true

How can we make the typeof() operator works in the way as we expected?
As we know when we create a object, the special property of the object constructor will be set to the function that create the object. which means:

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(new Date('2012-1-1')).constructor // Returns [Function Date]

So ideally we can retrieve the name of the function as the type of the variable. And to be compatible with javascript’s native operator, we need to convert the name to lower case. So we got this expression:

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function typeOf(obj) { // Use capital O to differentiate this function from typeof operator
	return obj ? obj.constructor.name.toLowerCase() : typeof(obj);
}
typeOf(new Date('2012-1-1')) === 'date' // Returns true

And luckily, we can also apply this to other primitive types, e.g:

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typeOf(123) === 'number'; // Returns true
typeOf('hello') === 'string'; // Returns true
typeOf(function(){}) === 'function';  // Returns true

or even

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typeOf({}) === 'object'; // Returns true

So in general, we use this expression as new implementation of the typeof() operator! EXCEPT One case!

If someone declare the object constructor in this way, our new typeof() implementation will work improperly!

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var SomeClass = (function() {
	return function() {
		this.someProperty='some value';
	}
})();

or even define the constructor like this

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var SomeClass = function() {
	this.someProperty = 'some value';
}

And we will find that

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typeOf(new SomeClass) === ''; // Returns true

the reason behind this is because the real constructor of the SomeClass is actually an anonymous function, whose name is not set.

To solve this problem, we need to declare the name of the constructor:

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var SomeClass = (function() {
	return function SomeClass() {
		this.someProperty='some value';
	}
})();

or even define the constructor like this

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var SomeClass = function SomeClass() {
	this.someProperty = 'some value';
}

The most common task when operating the servers is to distribute a file to multiple servers.
So I wrote a piece of shell script to solve this problem:

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#!/bin/bash  
echo "mscp <source file> <target dir>"  
SourceFile=$1  
TargetDir=$2  
echo "Copy $SourceFile to $TargetDir as $RemoteUser"  
echo "Enter the servers:"  
if [-f $SourceFile ]  
then  
  printf "File found, preparing to transfer\n"  
  while read server  
  do  
  scp -p $SourceFile ${server}:$TargetDir  
  done  
else  
  printf "File \"$SourceFile\"not found\n"  
  exit 1  
fi  
exit 0

call the script mscp <source file> <target dir>, then the script will ask you the list of target servers. So you can type them one by one. If the remote user is different than you current user, you can also explicitly identify it by typeing user@server

Beside the previous scenario, there is a more common sceanrio, that you have got a server list stored in afile already. Then instead of type the servers line by line, you can pipe the file content to the script.
e.g:

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cat server_list.txt > mscp src_files dest_path

I downloaded the TextMate bundle for node.js.
But this bundle doesn’t work properly.
When i clicked cmd+R to run javascript, it reports that it cannot get variable “TM_FILE” from undefined.
And the console output contains a warning that module “sys” is renamed as “util”.

To fix this two issues:
Some fix these 2 issues, some modification to command script is needed:

1. Open Command Editor in TextMate, and edit the command script of “Run File or Spec” under “JavaScript Node” category:
2. Change var sys = require("sys"); to var sys = require("util"); to fix the warning.
3. Replace all instances of process.ENV. with process.env.

After modification, close the Bundle Editor. Then ask TextMate to reload all the bundles.
Then the command will work perfectly now.

There is another Trick, since there had been a bundle called javascript in TextMate. So this node js bundle doesn’t activated when you editing .js file.
You need to press ctrl + alt + n to activate the bundle manually.

This problem can be fixed by changing scope selector of all the snippets and commands. You can change it from “source.js.node” to “source.js”