闭包是什么？

closure.js

function testClosure(paramClosure) {
	var innerClosure = 100;

	return function() {
		paramClosure++;
		innerClosure--;
	}

}

var retFunc = testClosure(0); //形成（或产生）闭包，包含一个可执行的函数，还有这个函数的外部变量（不是全局变量）
retFunc(); //paramClosure, innerClosure 两个变量不会因为retFunc结束而消失

//为什么要使用closure

//example：写一个counter
//no closure version
var cnt = 0;  //this is a global variable, urgly and buggy！！！！

function counter() {
	cnt++;
	return cnt;
}
var c1 = counter(); //record once
var c2 = counter(); //record record twice
console.log(c1, c2);


//use closure version
function getCounter() {
	var cnt = 0;
	return function() {
		cnt++;
		return cnt;
	}
}
var counter = getCounter();
var c1 = counter(); //record once
var c2 = counter(); //record record twice
console.log(c1, c2);

//example （lexical scope） 词法作用域
//闭包的实质就是变量作用域的问题
//一个变量的作用域总是和function产生关系时候，才变得有意义
//return 回来的不一定是function
function testScope() {
	var cnt1 = 0;
	var cnt2 = 0;
	var obj = {
		p:1
	};
	return {
		c1: function() {
			cnt1++;
			return cnt1;
		},
		c2: function() {
			cnt2 = cnt2+2;
			return cnt2;
		},
		obj:obj
	}
}

//test
var ret = testScope();
var c1 = ret.c1();
var c2 = ret.c1();
var c3 = ret.c2();
var c4 = ret.c2();
console.log(c1, c2, c3, c4, ret);

closure可以理解为一个只有私有变量类的一个实例