When the code is running asynchronously, the for-loop will end before the value 'fi' is printed.As a result of closure,inner function 'testClosure' has access to variables fi of the outer function .But the value of 'fi' will not be set to 0 any more(the first for-loop is finished). So the output of the asynchronous code is 1-2-3-4-5-6.

Test Javasript Closure.js

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

//http://rainsilence.iteye.com/blog/1534720
var sleep = function(time) {
	var startTime = new Date().getTime();
	while(new Date().getTime() < startTime + time);
};

//non-blcoking
var tevent = function() {
}
util.inherits(tevent,events.EventEmitter);

tevent.prototype.doSomething = function(){
	var eve = this;
	setTimeout(function(){
		eve.emit('end');
	},200);
}

tevent.prototype.Start = function(){
	this.emit('start');
}

function testClosure(callback) {
	var t = new tevent;
	t.on('start',function(){
		t.doSomething();
	}).on('end',function(){
		if (callback) {
			callback();
		};
	});
	t.Start();
};

//out: 123 123
(function(){
	console.log('Sync:');
	for(var i = 0; i < 2; ++i) {
		var fi = 0;
		for(var j = 0; j < 3; ++j) {
			sleep(200);
			++fi;
			console.log(fi);
		}
	}
})();

//out: 123 456
(function(){
	console.log('Async:');
	for(var i = 0; i < 2; ++i) {
		var fi = 0;
		for(var j = 0; j < 3; ++j) {
			testClosure(function(){
				++fi;
				console.log(fi);
			});
		}
	}
})();