JavaScript闭包的9个典型应用场景详解

//1.返回值 最常用的
    function fn(){
        var name="hello";
        return function(){
            return name;
        }
    }
    var fnc = fn();
    console.log(fnc())//hello

var fn2;
function fn(){
    var name="hello";
    //将函数赋值给fn2
    fn2 = function(){
        return name;
    }
}
fn()//要先执行进行赋值，
console.log(fn2())//执行输出fn2

function fn(){
    var name="hello";
    return function callback(){
        return name;
    }
}
var fn1 = fn()//执行函数将返回值（callback函数）赋值给fn1，
 
function fn2(f){
    //将函数作为参数传入
    console.log(f());//执行函数，并输出
}
fn2(fn1)//执行输出fn2

(function(){
        var name="hello";
        var fn1= function(){
            return name;
        }
        //直接在自执行函数里面调用fn2，将fn1作为参数传入
        fn2(fn1);
    })()
    function fn2(f){
        //将函数作为参数传入
        console.log(f());//执行函数，并输出
    }

//每秒执行1次，分别输出1-10
for(var i=1;i<=10;i++){
    (function(j){
        //j来接收
        setTimeout(function(){
            console.log(j);
        },j*1000);
    })(i)//i作为实参传入
}

function fn(){
        var name='hello'
        setName=function(n){
            name = n;
        }
        getName=function(){
            return name;
        }
         
        //将setName，getName作为对象的属性返回
        return {
            setName:setName,
            getName:getName
        }
    }
    var fn1 = fn();//返回对象，属性setName和getName是两个函数
    console.log(fn1.getName());//getter
        fn1.setName('world');//setter修改闭包里面的name
    console.log(fn1.getName());//getter

var arr =['aa','bb','cc'];
function incre(arr){
    var i=0;
    return function(){
        //这个函数每次被执行都返回数组arr中 i下标对应的元素
         return arr[i++] || '数组值已经遍历完';
    }
}
var next = incre(arr);
console.log(next());//aa
console.log(next());//bb
console.log(next());//cc
console.log(next());//数组值已经遍历完

var fn = (function(){
               var arr=[];//用来缓存的数组
                   return function(val){
                       if(arr.indexOf(val)==-1){//缓存中没有则表示需要执行
                           arr.push(val);//将参数push到缓存数组中
                           console.log('函数被执行了',arr);
                           //这里写想要执行的函数
                       }else{
                           console.log('此次函数不需要执行');
                       }
                       console.log('函数调用完打印一下，方便查看已缓存的数组：',arr);
                   }
               })();
        
       fn(10);
       fn(10);
       fn(1000);
       fn(200);
       fn(1000);

//比如求和操作，如果没有缓存，每次调用都要重复计算，采用缓存已经执行过的去查找，查找到了就直接返回，不需要重新计算
      
     var fn=(function(){
        var cache={};//缓存对象
        var calc=function(arr){//计算函数
            var sum=0;
            //求和
            for(var i=0;i<arr.length;i++){
                sum+=arr[i];
            }
            return sum;
        }
         
        return function(){
            var args = Array.prototype.slice.call(arguments,0);//arguments转换成数组
            var key=args.join(",");//将args用逗号连接成字符串
            var result , tSum = cache[key];
            if(tSum){//如果缓存有   
                console.log('从缓存中取：',cache)//打印方便查看
                result = tSum;
            }else{
                //重新计算，并存入缓存同时赋值给result
                result = cache[key]=calc(args);
                console.log('存入缓存：',cache)//打印方便查看
            }
            return result;
        }
     })();
    fn(1,2,3,4,5);
    fn(1,2,3,4,5);
    fn(1,2,3,4,5,6);
    fn(1,2,3,4,5,8);
    fn(1,2,3,4,5,6);