闭包

1、闭包的概念

当内部函数被保存到外部时，将会生成闭包。闭包会导致原有作用域链不释放，造成内存泄漏（占用得越多，剩下的内存越少）。

2、闭包的作用

a:实现公有变量（函数累加器）

 function test(){
            var num = 100;
            function a(){
                num++;
                console.log(num);
            }
            function b(){
                num--;
                console.log(num);
            }
            return[a,b]
        }
        var myArr = test()
        myArr[1]();//99
        myArr[0]();//100

b:可以做缓存（存储结构）

function test(){
            var food = 'apple';
            var obj = {
                eatFood:function(){
                    if(food !==''){
                        console.log('I am eating' + food);
                        food = '';
                    }else{
                        console.log('There is nothing!')
                    }
                },
                pushFood:function(myFood){
                    food = myFood
                }
            }
            return obj;
        }
        var person = test();
        person.eatFood();//I am eating apple
        person.eatFood();//There is nothing!
        person.pushFood('banana');
        person.eatFood();//I am eating banana

c:可以实现封装，属性私有化

function Fan(name,wife){
  var prepareWife = 'xiaowu';
  this.name = name; 
  this.wife = wife;
  this.divorce = function(){
     this.wife =  prepareWife 
  }   
  this.changePrepareWife = function(target){
    prepareWife = target
  }
  this.sayPreparewife = function(){
    console.log(prepareWife)
  }
}

var fan = new Fan('deng','xiaoyou')

 

d:模块化开发，防止污染全局变量

3、立即执行函数（针对初始化功能的函数）

只有表达式才能被执行符号执行

4、未经声明的变量只有放在typeof之后才不会报错

5、只有表达式才能被执行

+function test(){
   console.log('a')  
}

(function(){
    console.log('a')
}())

 function test(){
            var arr = [];
            for(var i = 0; i<10; i++){
                arr[i]=function(){
                    console.log(i)//10个10
                }
            }
            return arr;
        }
        var myArr = test();
        for(var j=0;j<10;j++){
            myArr[j]()
        }
 function test(){
            var arr = [];
            for(var i = 0; i<10; i++){
               (function(j){
                   document.write(j+'')//1到9
               }(i))
            }
            return arr;
        }
        var myArr = test();
        for(var j=0;j<10;j++){
            myArr[j]()
        }